Help buddy for his amputation!

I have a question. I'm currently in trouble financially. I rescued a cat about 2 years ago. His name is buddy. He bonded into my home very quickly. When he was a few months old he got very ill. I rushed him to the rescuer who specializes in cats and she saved him. He was basically dead as we believe he had some type of flu. Every since that happened he was unable to walk properly. He always stumbled and is very clumsy. Well he broke his foot a while back. I rushed him to the vet who did X-rays and put him in a cast for 8 weeks. And even though I work with animals I was afraid to do the cast change each week so I paid every week to have it rewrapped. Not even a few months later he broke it again. I rushed him back to the vet and they kept him over night. Did more X-rays and administrated the highest pain meds for cats. For the next 8 weeks I kept returning him to the vet to get a cast changed and more pain meds. Not even a week later after the second cast came off I noticed a lump on his foot. He was growling and in pain. So once again I rushed him to the vet. They kept him again over night did more X-rays and gave him pain meds. They said they believe it's a bone infection or cancer. I took him for 3 vet opinions and they all said the same thing. That the leg must be amputated immediately. I spent well over 2 grand and maxed out my credit card in order to ensure my cat is well taken care of. I got quoted for 3 different vets how much the amputation will be. First vet said 2400$. Second vet said 2200. And third vet who a friend recommended to try quoted me today 900. Which is a huge difference compared to the other vets. I financially can't afford the 900$ for the amputation. I spoke to a few chairtys who I sent records of all the vet expenses I paid and my current year to date makings. I do not make enough. The chairtys told me there behind and on a 6 month waiting list. I'm just so heart broken and don't know what else to do. Can anyone help? I have proof of the expenses that it will cost for amputation. Please I'm desperate and he is in pain. It breaks my heart. He is still a baby. Attached is a picture of him. Thank you and God bless