Have another look at the drop down menu beside the item, there should be an option that says "mark as not sent"  then unmark as payment received, I just checked a couple of paid and sent items, they even have tracking info attached, but the option was there had I need it,   if they work, resend the invoice, the buyer will then be able to pay again.

Hopefully you can sort it out with the buyer,  see what they say, it may have been an issue with their PP account or credit card funds.