I would bet you can sue them for property damage without messing with the ASPCA. That horse was your property and they destroyed its value before they had it paid for. Contact an attorney if you want to persue it.
Also for what its worth I would geld the horse once he was healthier. Geldings, in my opinion, have a much better life than stallions. They generally are calmer and can socialize much easier than stallions. If you wanted to keep him for income then that would be one thing but since you want to lease him out you won't likely make any money on him. Geld him and sell him to a good home or keep him and ride him.
There's nothing like the Rockies in the springtime... Nothing like the freedom in the air... And there ain't nothing better than draggin calves to the fire and there's nothing like the smell of burning hair. -Brenn Hill