apply journal entry to open invoice

I have a couple old invoices that were not cleared correctly when the payments came in.  They show as open but they are not.  I have created journal entries to credit A/R and debit the appropriate account and have correctly assigned these to the customer.  I then go in to the invoice, click on recieve payment and see both the invoices and the journal entries (under credits).  The amount totals zero.  When I try to save/close I get an error message that says "You must set a transaction amount".  From my research in the community help and support help I should be able to offset the open invoices with the credits - so why does it not let me save?  I don't have an amount to enter as the credits completely offset the open invoices amounts.  I cannot figure out what I am doing wrong.  Any help would be greatly appreciated.  Thanks