You can write anything you want in a sales agreement and 99% of the time, it is perfectly legal.
This is what I put in mine:
Seller(s) and Buyer(s) name and address. Name and description of horse (breed, height, color, gender, registration info if any)
Terms of the sale.
Date and sign.
It sounds like you are going to have a trial. If so, then state the length of the trial, what the horse can/cannot do during the trial, and the deadline for notification if the horse is unacceptable. Also, indicate how long you would have to make a refund. I will do it within 5 business days, for example.
I would not let a horse leave my premises without full payment and a PPE done prior to departure. I also like to keep trials short. I have no issue with agreeing to a full refund, but you want to be sure you are getting the same horse back if it doesn't work out. But if I am pulling my horse off the market for a sale that potentially might not happen, I want the trial to be short.