My cat got hit and his leg broken. What should I do?

My cat ran out the door into the street tonight and got hit. I took her to the vet and he said her leg was broken at the growth plate. He said he could do surgery but it might not be successful because of the damage. The surgery would be around 600 dollars which I can not do. He said he could amputate it for 300 dollars. He said I would have to pay half down before he did the surgery and I could pay the rest off. I might be able to get the 150 for amputation so should I do that?