Invoice Line - Error: You must have a receipt number

We have a credit memo invoice where the first line has been discarded and a second line added and matched.  We are now trying to discard the 2nd line, but get an error message saying "You must have a receipt number."  It will not let us discard and save.  There is no receipt number showing up on the line, but if you go to the distribution it is there.  The issue arose because after creating the credit memo and matching originally we thought that the amount billed should decrease, but it did not.  I'm guessing this also has something to