I sent a broodmare to a farm to foal. I really didn't want to breed anymore, and the farm owners loved the mare, so I gave her to them for a years board for the baby after it was weaned. It worked out great for both of us.
I did that with an old car... my BO was in need of an inexpensive runaround town car, I was getting a new car so rather than trade mine in I offered her slightly less than blue book in exchange for X number of months board. Worked out well for both of us.
In your case I would have a Bill of Sale which included the sale prize of say $6,000 which will be paid by and in exchange for board of $500 per month for 1 year.
I would also include should you no longer wish to board horse for any reason, buyer has X days to pay any remaining balance of purchase not offset by boarding fees; and include some protection in case of injury or death of horse