entering vendor credit bills

I entered the amount of the credit in the upper portion of the bill.  Then when I did the account breakdown, I added the information form the invoice.  The lower portion equalled the upper portion but when I tried to save it, I got the message that the bill was not in balance.  I double checked the adding forward and backward and it is indeed balanced.  However, the program will not let me move on because it will not save the credit bill.  Please help.