how can I fix a/c on 9700 Ford tractor?

I had my 9700 Ford tractor air conditioner worked on.
They put on a generic compressor and converted it to
the new style of freon. The compressor belt comes off
every time I have tried to use it. They have added
home made brackets to try to keep it on. Nothing ever
works for more than a few minutes. Now they are telling me that they have done all they can and so I am
stuck with an a/c that doesn't work and am out over $2,200. Can anybody tell me what to do?