Man, tough situation. I wouldn't let $250 slide, let alone $2500.
Absolutely make this couple's life a living Hell. I can not stand people verbally saying one thing and then screwing people in writing.
I have kept deposits on cars that buyers did not follow through on. But, it was repeated multiple time verbally and in writing with a witness.
I like the next step of showing up at their house with a Statey. But, as said before, someone who has the balls to take your 2500 after saying they would not probably would just be jackasses to your father as well.