Help for Peanut

My moms dog Peanut is in need of some medical care. Left rear leg is giving her a lot of problems it buckles out from under so much that she will barely walk and she is in pain. She has arthritis pretty bad and has been on steroids in the last month already for it. 2 weeks ago she was trying to get  up in my moms chair and she slipped of and landed on this leg and instantly fell and cried with the pain. I have done everything I could for her. She even allowed me to massage and flex her leg. I am not so sure if she didn't fracture it. There is no visible abnormality of the leg until she walks on and it buckles on her.  She now will not move from my moms lap. My mom lives with me and my son since my dad passed away 3 years ago. Her only income is social security and neither of us can afford to get Peanut the medical care she needs. Just the office visit and xray are $175. And I am almost certain she will need more then that. 3 weeks ago I took her to the vet for my mom and it cost me $350 for a full body xray and to find out her trechia is collapsing and causing her breathing difficulties and a cough. I just cant afford more to help with this one.
Peanut is my moms life support since my dad passed. They were married for 56 years and I would hate to see her lose Peanut.

Thank you for taking your time to read this and bless you for your consideration for helping my mom and her fur baby Peanut.