Please help my dog get surgery

Dahlia was abandoned by her previous owner. I was pet sitting for her and she just never came back for Dahlia. Dahlia is barely four years old and has been limping for at least a year. She hurt her leg when she broke away from me on a walk to chase a squirrel. The doctor says that she needs surgery on the knee on her back leg. However, I cannot afford the surgery. We have been putting a "Band-Aid" on the situation by giving her pain medicines and anti-inflammatory medications. But the medicines make her so tired. She is not an old dog but she is acting like it due to the effects of the medicine and the pain of the bad leg. 

Now, it seems as though even the medicine is not helping. Surgery is Dahlia's only option for having a good quality of life. Please help. No amount is too small. She cannot walk and play normally. She has gained so much weight because she is unable to exercise. Please help!!!!