wrong diagnosis and not returning check

We had a & E out and they diagnosed a refrigerator compressor, Unfortunately we were in NY as my husband was having major surgery and just my mother was there. They needed a check for the parts beforehand so my mom paid them in advance.
When we came home the next day my brother had plugged the ref in and it has worked since that is over 2 months. They HAVE NOT returned our money event though we returned the parts to them.

We are now going to sue them in small claims court.