Bottom line is you can't. Have you tried talking to a vet about making payments, or looked for a low-cost animal clinic that can help your cat? It's not fair to make your cat suffer because you can't afford a vet. I am not trying to be "mean" but rather than have your cat suffer, if you can't possibly find any way to get it medical attention, maybe you should consider the Humane Society. Maybe they could help you with vet bills or worst case scenario, give her/him to them if they would agree to fix the leg and adopt him/her back out. I hope you find a way to get your cat help. Good luck.