Oh dear! Too bad your vet or rescue didn't find his microchip at the beginning. I'm just wondering though if the owners "dumped" him knowing that he had a medical condition. I would definitely turn the receipts into your rescue, didn't he come through One By One? If so, then that's what they are there for!!! They shouldn't be able to pick and choose which ones they fund!

Hugs and Prayers!
Kim