Crixus leg amputation

Hi I’m Crixus! I’m 4 months, almost 5. I was adopted July 29th to a great home.  During birth my umbilical cord got wrapped around my foot and unfortunately snapped off most of my left hind leg off. It wasn’t attended to properly and caused me so much discomfort that i chewed it down to the bone and now it’s infected:( I’m in an enormous amount of pain and The vet says i have to cut it past my knee now and it’s not gonna be cheap. Please help me so I can feel like a normal kitten again:)