diff --git a/org.adempiere.base/src/org/adempiere/exceptions/AverageCostingNegativeQtyException.java b/org.adempiere.base/src/org/adempiere/exceptions/AverageCostingNegativeQtyException.java
new file mode 100644
index 0000000000..0421e5b606
--- /dev/null
+++ b/org.adempiere.base/src/org/adempiere/exceptions/AverageCostingNegativeQtyException.java
@@ -0,0 +1,36 @@
+/******************************************************************************
+ * Product: iDempiere ERP & CRM Smart Business Solution                       *
+ * Copyright (C) 2012 Heng Sin Low                							  *
+ * Copyright (C) 2012 www.iDempiere.org                							  *
+ * This program is free software; you can redistribute it and/or modify it    *
+ * under the terms version 2 of the GNU General Public License as published   *
+ * by the Free Software Foundation. This program is distributed in the hope   *
+ * that it will be useful, but WITHOUT ANY WARRANTY; without even the implied *
+ * warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.           *
+ * See the GNU General Public License for more details.                       *
+ * You should have received a copy of the GNU General Public License along    *
+ * with this program; if not, write to the Free Software Foundation, Inc.,    *
+ * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.                     *
+ *****************************************************************************/
+package org.adempiere.exceptions;
+
+/**
+ * 
+ * @author hengsin
+ *
+ */
+public class AverageCostingNegativeQtyException extends AdempiereException {
+
+	/**
+	 * generated serial version id
+	 */
+	private static final long serialVersionUID = 4165497320719149773L;
+
+	public AverageCostingNegativeQtyException() {
+		super();
+	}
+
+	public AverageCostingNegativeQtyException(String message) {
+		super(message);
+	}
+}
diff --git a/org.adempiere.base/src/org/compiere/acct/Doc.java b/org.adempiere.base/src/org/compiere/acct/Doc.java
index e463585776..84387501f0 100644
--- a/org.adempiere.base/src/org/compiere/acct/Doc.java
+++ b/org.adempiere.base/src/org/compiere/acct/Doc.java
@@ -28,6 +28,7 @@ import java.util.Iterator;
 import java.util.Properties;
 import java.util.logging.Level;
 
+import org.adempiere.exceptions.AverageCostingNegativeQtyException;
 import org.compiere.model.MAccount;
 import org.compiere.model.MAcctSchema;
 import org.compiere.model.MConversionRate;
@@ -516,6 +517,12 @@ public abstract class Doc
 				p_Status = postLogic ();
 			}
 		}
+		catch (AverageCostingNegativeQtyException e)
+		{
+			log.log(Level.INFO, e.getLocalizedMessage(), e);
+			p_Status = STATUS_NotPosted;
+			p_Error = e.toString();
+		}
 		catch (Exception e)
 		{
 			log.log(Level.SEVERE, "", e);
@@ -525,7 +532,7 @@ public abstract class Doc
 
 		String validatorMsg = null;
 		// Call validator on before post
-		if (!p_Status.equals(STATUS_Error)) {
+		if (p_Status.equals(STATUS_Posted)) {
 			validatorMsg = ModelValidationEngine.get().fireDocValidate(getPO(), ModelValidator.TIMING_BEFORE_POST);
 			if (validatorMsg != null) {
 				p_Status = STATUS_Error;
@@ -536,7 +543,7 @@ public abstract class Doc
 		//  commitFact
 		p_Status = postCommit (p_Status);
 
-		if (!p_Status.equals(STATUS_Error)) {
+		if (p_Status.equals(STATUS_Posted)) {
 			validatorMsg = ModelValidationEngine.get().fireDocValidate(getPO(), ModelValidator.TIMING_AFTER_POST);
 			if (validatorMsg != null) {
 				p_Status = STATUS_Error;
diff --git a/org.adempiere.base/src/org/compiere/model/MCost.java b/org.adempiere.base/src/org/compiere/model/MCost.java
index 9b53af0cd7..5c524ba763 100644
--- a/org.adempiere.base/src/org/compiere/model/MCost.java
+++ b/org.adempiere.base/src/org/compiere/model/MCost.java
@@ -27,6 +27,7 @@ import java.util.List;
 import java.util.Properties;
 import java.util.logging.Level;
 
+import org.adempiere.exceptions.AverageCostingNegativeQtyException;
 import org.adempiere.exceptions.DBException;
 import org.compiere.Adempiere;
 import org.compiere.util.CLogger;
@@ -1458,6 +1459,12 @@ public class MCost extends X_M_Cost
 		{
 			amt = amt.multiply(new BigDecimal(-1.00d));
 		}
+		
+		if (getCurrentQty().add(qty).signum() < 0)
+		{
+			throw new AverageCostingNegativeQtyException("Product(ID)="+getM_Product_ID()+", Current Qty="+getCurrentQty()+", Trx Qty="+qty);
+		}
+		
 		BigDecimal oldSum = getCurrentCostPrice().multiply(getCurrentQty());
 		BigDecimal newSum = amt;	//	is total already
 		BigDecimal sumAmt = oldSum.add(newSum);