diff --git a/htdocs/compta/facture/class/facture.class.php b/htdocs/compta/facture/class/facture.class.php
index e302c83e2d8..4ae9fe197fe 100644
--- a/htdocs/compta/facture/class/facture.class.php
+++ b/htdocs/compta/facture/class/facture.class.php
@@ -5171,11 +5171,11 @@ class FactureLigne extends CommonInvoiceLine
 		// Check parameters
 		if ($this->product_type < 0) return -1;
 
-		// if buy price not defined, define buyprice as configured in margin admin
-		if ($this->pa_ht == 0 && $pa_ht_isemptystring)
-		{
-			if (($result = $this->defineBuyPrice($this->subprice, $this->remise_percent, $this->fk_product)) < 0)
-			{
+		// if buy price not provided, define buyprice as configured in margin admin
+		if ($this->pa_ht == 0 && $pa_ht_isemptystring) {
+			// We call defineBuyPrice only if data was not provided (if input was '0', we will not go here and value will remaine '0')
+			$result = $this->defineBuyPrice($this->subprice, $this->remise_percent, $this->fk_product);
+			if ($result < 0) {
 				return $result;
 			} else {
 				$this->pa_ht = $result;
@@ -5214,7 +5214,7 @@ class FactureLigne extends CommonInvoiceLine
 			$sql .= ", total_localtax2=".price2num($this->total_localtax2);
 		}
 		$sql .= ", fk_product_fournisseur_price=".(!empty($this->fk_fournprice) ? "'".$this->db->escape($this->fk_fournprice)."'" : "null");
-		$sql .= ", buy_price_ht='".price2num($this->pa_ht)."'";
+		$sql .= ", buy_price_ht=".(($this->pa_ht || $this->pa_ht === 0 || $this->pa_ht === '0') ? price2num($this->pa_ht) : "null");	// $this->pa_ht should always be defined (set to 0 or to sell price depending on option)
 		$sql .= ", fk_parent_line=".($this->fk_parent_line > 0 ? $this->fk_parent_line : "null");
 		if (!empty($this->rang)) $sql .= ", rang=".$this->rang;
 		$sql .= ", situation_percent=".$this->situation_percent;
diff --git a/htdocs/core/class/commonobject.class.php b/htdocs/core/class/commonobject.class.php
index af2c902aacb..8a3f50c02a4 100644
--- a/htdocs/core/class/commonobject.class.php
+++ b/htdocs/core/class/commonobject.class.php
@@ -7122,7 +7122,7 @@ abstract class CommonObject
 
 		$buyPrice = 0;
 
-		if (($unitPrice > 0) && (isset($conf->global->ForceBuyingPriceIfNull) && $conf->global->ForceBuyingPriceIfNull == 1)) // In most cases, test here is false
+		if (($unitPrice > 0) && (isset($conf->global->ForceBuyingPriceIfNull) && $conf->global->ForceBuyingPriceIfNull > 0)) // In most cases, test here is false
 		{
 			$buyPrice = $unitPrice * (1 - $discountPercent / 100);
 		} else {
diff --git a/htdocs/core/class/html.formmargin.class.php b/htdocs/core/class/html.formmargin.class.php
index 233f98df662..608698cf1b1 100644
--- a/htdocs/core/class/html.formmargin.class.php
+++ b/htdocs/core/class/html.formmargin.class.php
@@ -91,8 +91,10 @@ class FormMargin
 				if ($product->fetch_product_fournisseur_price($line->fk_fournprice))
 					$line->pa_ht = $product->fourn_unitprice * (1 - $product->fourn_remise_percent / 100);
 			}
-			// si prix d'achat non renseigné et devrait l'être, alors prix achat = prix vente
-			if ((!isset($line->pa_ht) || $line->pa_ht == 0) && $line->subprice > 0 && (isset($conf->global->ForceBuyingPriceIfNull) && $conf->global->ForceBuyingPriceIfNull == 1)) {
+
+			// If buy price is not defined (null), we will use the sell price. If defined to 0 (it means it was forced to 0 during insert, for example for a free to get product), we must still use 0.
+			//if ((!isset($line->pa_ht) || $line->pa_ht == 0) && $line->subprice > 0 && (isset($conf->global->ForceBuyingPriceIfNull) && $conf->global->ForceBuyingPriceIfNull > 0)) {
+			if ((!isset($line->pa_ht)) && $line->subprice > 0 && (isset($conf->global->ForceBuyingPriceIfNull) && $conf->global->ForceBuyingPriceIfNull > 0)) {
 				$line->pa_ht = $line->subprice * (1 - ($line->remise_percent / 100));
 			}
 
diff --git a/htdocs/langs/en_US/margins.lang b/htdocs/langs/en_US/margins.lang
index 76ea8ad5c4d..c3bff2305de 100644
--- a/htdocs/langs/en_US/margins.lang
+++ b/htdocs/langs/en_US/margins.lang
@@ -22,7 +22,7 @@ ProductService=Product or Service
 AllProducts=All products and services
 ChooseProduct/Service=Choose product or service
 ForceBuyingPriceIfNull=Force buying/cost price to selling price if not defined
-ForceBuyingPriceIfNullDetails=If buying/cost price not defined, and this option "ON", margin will be zero on line (buying/cost price = selling price), otherwise ("OFF"), marge will be equal to suggested default.
+ForceBuyingPriceIfNullDetails=If buying/cost price not provided when we add a new line, and this option is "ON", the margin will be 0 on the new line (buying/cost price = selling price). If this option is "OFF", margin will be equal to the value suggested by default (and 100% if no default value can be found).
 MARGIN_METHODE_FOR_DISCOUNT=Margin method for global discounts
 UseDiscountAsProduct=As a product
 UseDiscountAsService=As a service
diff --git a/htdocs/margin/agentMargins.php b/htdocs/margin/agentMargins.php
index e7229abd2d3..d2a19e87149 100644
--- a/htdocs/margin/agentMargins.php
+++ b/htdocs/margin/agentMargins.php
@@ -170,7 +170,11 @@ if (!empty($startdate))
 if (!empty($enddate))
   $sql .= " AND f.datef <= '".$db->idate($enddate)."'";
 $sql .= " AND d.buy_price_ht IS NOT NULL";
-if (isset($conf->global->ForceBuyingPriceIfNull) && $conf->global->ForceBuyingPriceIfNull == 1) $sql .= " AND d.buy_price_ht <> 0";
+// We should not use this here. Option ForceBuyingPriceIfNull should have effect only when inserting data. Once data is recorded, it must be used as it is for report.
+// We keep it with value ForceBuyingPriceIfNull = 2 for retroactive effect but results are unpredicable.
+if (isset($conf->global->ForceBuyingPriceIfNull) && $conf->global->ForceBuyingPriceIfNull == 2) {
+	$sql .= " AND d.buy_price_ht <> 0";
+}
 //if ($agentid > 0) $sql.= " GROUP BY s.rowid, s.nom, s.code_client, s.client, u.rowid, u.login, u.lastname, u.firstname";
 //else $sql.= " GROUP BY u.rowid, u.login, u.lastname, u.firstname";
 $sql .= " GROUP BY s.rowid, s.nom, s.code_client, s.client, u.rowid, u.login, u.lastname, u.firstname";
diff --git a/htdocs/margin/customerMargins.php b/htdocs/margin/customerMargins.php
index 468d633875d..feb5cd34c8e 100644
--- a/htdocs/margin/customerMargins.php
+++ b/htdocs/margin/customerMargins.php
@@ -234,8 +234,11 @@ $sql .= " AND f.datef >= '".$db->idate($startdate)."'";
 if (!empty($enddate))
 $sql .= " AND f.datef <= '".$db->idate($enddate)."'";
 $sql .= " AND d.buy_price_ht IS NOT NULL";
-if (isset($conf->global->ForceBuyingPriceIfNull) && $conf->global->ForceBuyingPriceIfNull == 1)
-$sql .= " AND d.buy_price_ht <> 0";
+// We should not use this here. Option ForceBuyingPriceIfNull should have effect only when inserting data. Once data is recorded, it must be used as it is for report.
+// We keep it with value ForceBuyingPriceIfNull = 2 for retroactive effect but results are unpredicable.
+if (isset($conf->global->ForceBuyingPriceIfNull) && $conf->global->ForceBuyingPriceIfNull == 2) {
+	$sql .= " AND d.buy_price_ht <> 0";
+}
 if ($client) $sql .= " GROUP BY s.rowid, s.nom, s.code_client, s.client, f.rowid, f.ref, f.total, f.datef, f.paye, f.fk_statut";
 else $sql .= " GROUP BY s.rowid, s.nom, s.code_client, s.client";
 $sql .= $db->order($sortfield, $sortorder);
diff --git a/htdocs/margin/productMargins.php b/htdocs/margin/productMargins.php
index ebe8c9c1cac..b7695c9b2ea 100644
--- a/htdocs/margin/productMargins.php
+++ b/htdocs/margin/productMargins.php
@@ -197,8 +197,11 @@ if (!empty($startdate))
 if (!empty($enddate))
   $sql .= " AND f.datef <= '".$db->idate($enddate)."'";
 $sql .= " AND d.buy_price_ht IS NOT NULL";
-if (isset($conf->global->ForceBuyingPriceIfNull) && $conf->global->ForceBuyingPriceIfNull == 1)
+// We should not use this here. Option ForceBuyingPriceIfNull should have effect only when inserting data. Once data is recorded, it must be used as it is for report.
+// We keep it with value ForceBuyingPriceIfNull = 2 for retroactive effect but results are unpredicable.
+if (isset($conf->global->ForceBuyingPriceIfNull) && $conf->global->ForceBuyingPriceIfNull == 2) {
 	$sql .= " AND d.buy_price_ht <> 0";
+}
 if ($id > 0) $sql .= " GROUP BY p.label, p.rowid, p.fk_product_type, p.ref, p.entity, d.fk_product, f.rowid, f.ref, f.total, f.datef, f.paye, f.fk_statut";
 else $sql .= " GROUP BY p.label, p.rowid, p.fk_product_type, p.ref, p.entity";
 $sql .= $db->order($sortfield, $sortorder);
diff --git a/htdocs/margin/tabs/productMargins.php b/htdocs/margin/tabs/productMargins.php
index 5fcec11c93b..eb82f121295 100644
--- a/htdocs/margin/tabs/productMargins.php
+++ b/htdocs/margin/tabs/productMargins.php
@@ -150,7 +150,11 @@ if ($id > 0 || !empty($ref))
 			if (!$user->rights->societe->client->voir && !$socid) $sql .= " AND s.rowid = sc.fk_soc AND sc.fk_user = ".$user->id;
 			if (!empty($socid)) $sql .= " AND f.fk_soc = $socid";
 			$sql .= " AND d.buy_price_ht IS NOT NULL";
-			if (isset($conf->global->ForceBuyingPriceIfNull) && $conf->global->ForceBuyingPriceIfNull == 1) $sql .= " AND d.buy_price_ht <> 0";
+			// We should not use this here. Option ForceBuyingPriceIfNull should have effect only when inserting data. Once data is recorded, it must be used as it is for report.
+			// We keep it with value ForceBuyingPriceIfNull = 2 for retroactive effect but results are unpredicable.
+			if (isset($conf->global->ForceBuyingPriceIfNull) && $conf->global->ForceBuyingPriceIfNull == 2) {
+				$sql .= " AND d.buy_price_ht <> 0";
+			}
 			$sql .= " GROUP BY s.nom, s.rowid, s.code_client, f.rowid, f.ref, f.total, f.datef, f.paye, f.fk_statut, f.type";
 			if (!$user->rights->societe->client->voir && !$socid) $sql .= ", sc.fk_soc, sc.fk_user";
 			$sql .= $db->order($sortfield, $sortorder);
diff --git a/htdocs/margin/tabs/thirdpartyMargins.php b/htdocs/margin/tabs/thirdpartyMargins.php
index bfa5ac15811..0b79ec62b90 100644
--- a/htdocs/margin/tabs/thirdpartyMargins.php
+++ b/htdocs/margin/tabs/thirdpartyMargins.php
@@ -163,7 +163,11 @@ if ($socid > 0)
 	$sql .= " AND d.fk_facture = f.rowid";
 	$sql .= " AND f.fk_soc = $socid";
 	$sql .= " AND d.buy_price_ht IS NOT NULL";
-	if (isset($conf->global->ForceBuyingPriceIfNull) && $conf->global->ForceBuyingPriceIfNull == 1) $sql .= " AND d.buy_price_ht <> 0";
+	// We should not use this here. Option ForceBuyingPriceIfNull should have effect only when inserting data. Once data is recorded, it must be used as it is for report.
+	// We keep it with value ForceBuyingPriceIfNull = 2 for retroactive effect but results are unpredicable.
+	if (isset($conf->global->ForceBuyingPriceIfNull) && $conf->global->ForceBuyingPriceIfNull == 2) {
+		$sql .= " AND d.buy_price_ht <> 0";
+	}
 	$sql .= " GROUP BY s.nom, s.rowid, s.code_client, f.rowid, f.ref, f.total, f.datef, f.paye, f.fk_statut, f.type";
 	$sql .= $db->order($sortfield, $sortorder);
 	// TODO: calculate total to display then restore pagination