So as stated before, it added the credit even though it displayed the error when I first tried to add credit to clients account then credit it on the invoice. After reading your message though, you are correct, that there was only one partial payment. I did not notice the "Amount to pay". This seems a bit confusing and maybe a bit redundant with there already being a slot for payment amount atop, which is probably why I overlooked it. This would probably fix my issue though. Thanks for explaining it! Things like this are always a pain when its a little thing..hehe.
Thanks again!