Even though this is a old post, I will go ahead and reply to it in hopes that it might help someone out there.
There are two ways to handle a credit from a vendor in regards to wrong pricing.
1) To retain proper costing information in inventory, you would need to run a Material Return under Purchase Order > Mat . Req/Return >Return of Goods Entry. Enter in your vendor then the inventory item and the quantity of materials along with the costs (all numbers entered will be positive numbers, the system will handle the the process of creating the negative amounts) that the vendor charged you originally. This will create a negative invoice amount under Accounts Payable for that vendor. Then go into Purchase Order > Main > Receipt of Goods Entry and enter in the same inventory item, quantity and the new corrected price. The difference in payables should be equal to the credit your vendor has provided.
2) If you are not worried about costing information in inventory, go into Accounts Payable > Main > Invoice Data Entry and enter in just the credit amount (negative dollar amount) that the vendor gave you. Select the same inventory GL account that the original material order was posted to. This will create a credit offset from the original invoiced amount.
Hope this helps!
