Help with Toby's leg

hi as many of you guys know, me and Lucas rescued this little guy from off the streets in January. Once we had him, we noticed he has a ton of issues walking and doesn't have very good balance at all. On Tuesday the 29th, Toby was playing with his sister Lily as they usually do, and he took a weird step and fell hard. Ever since then, he hasn't been able to walk or put any weight on that back left leg at all, and he's been stuck on the couch without being able to move, even as I'm typing this.

We took him to his vet and they checked it out and it isn't broken, but it's a neurological problem. Since we've had him, we've been told that he has neurological issues and was completely healthy other than just being a wobbly boy, until he hurt his leg. We've taken him to the specialist, and the cost of just the mri alone is going to be $6,000. It could be anything from an infection to a brain tumor, so once we can afford the mri, we have to worry about what to do after/how much it’ll be. Since Lucas and I bartend and serve, we aren't making nearly enough money to cover all these costs. We want Toby to be healthy and happy and for him and his sister to be able to play again like they used to, but we can't have that without going to this vet and figuring out what's wrong with him.