Help save dumbledor

Hi my name is Hannah, I am fundraising for a horse called Dumbledor. He is A beautiful gentle 14h grey 12yr gelding. Unfortunately he was Misold to us and has a bad knee injury. Advertised as a lovely all rounder we brought unseen (which I would never ever advise). Came off the lorry very dirty, prettified and on basically 3 legs. When we questioned and explained he was not fit for purpose the seller went on to say they would take him back on sales livery be riding him no problem! I could not let this happen he would never be sound enough to ride and I hate to think what would of happened. So he is here to stay. Losing already lots of money on Dumble, with ongoing vet bills, stabling and much more. My vet has concluded he’s happy being a glorified lawn mower but only option to fix his knee would be to operate. He will still never be able to be ridden but maybe a chance of a sound life in the field. My worry is without operation this will get much worse in winter. Now this isn’t cheap and my insurance won’t cover. So please if you could donate anything to get dumbledor the treatment he needs would be very much appreciated.