I guess this comes down to a question of what’s the right thing to do as opposed to what you’re obligated to do.
If the pro in question had the horse in the barn for 6 weeks and the sales activity was answering questions about an ad placed somewhere, then I think nothing more than “Thanks for your effort, we’ve decided not to sell.” The pro will continue to make make money off of your continued training board.
If, however, the pro in question does have some time and effort in the sales effort - worked on getting decent sales video and photos, wrote the ad, has been actively showing the horse to buyers, maybe hauled it off site to show to buyers, the type of activity that’s normally compensated by the commission, then I think the correct thing to do would be offer them some compensation, something short of the full commission. I would especially take this approach if you wanted to continue to work with this pro.
Again, the horse didn’t fail to sell and generate a commission because of a failure of effort on the pro’s part; the horse’s owners decided to take the horse off the market.
And yes, a contract that spelled out exactly what would happen in this situation would be a good idea.