Yeah... Crediting him for the fish you sold him is totally reasonable, but all his fish that you didn't sell him? Nice try.
It's totally common for fish to be carriers where they carry a parasite and are immune to it and show no signs themselves but can pass it on to a tank. Even the best care at a LFS
won't catch everything. It's up to the buyer to do a round of QT with parasite treatments prior to adding it to a tank. If you don't do that, you're responsible for the risks to your tank.