To enter a credit received from a vendor,just enter a payable with a negative amount and post it. You can then apply the credit against an open payable using Adjust A/P. However, if you want to process the credit without applying it to an open payable do the following:
1) Adjust A/P using negative amount and applied, use Suspense for offset account.
2) Transactions > Other Checks Received. Receive positive amounts, use Suspense for offset account again.