Interesting, I know that especially now people are trying to lease out their horses, as they don't want to sell them, but are having a hard time affording them, but to try and "lease" one out that is currently injured, and needs care above and beyond feeding and cleaning of stalls, and basic attention is just not right.
As for the mustang, has it had the halter on it for the last 3 years that its been in the pasture? Right off the bat that horse is going to cost a pretty penny, as I doubt that the horse has had its regular shots, farrier work, teeth will need to be done, and of course the time and resources to rig stuff up in the "large" pasture so that the person MIGHT have a better chance of catching the horse, and then of course who knows whether or not the horse loads into a trailer. I do have to say, at least the guy isn't trying to sell the horse to an unsuspecting buyer. I would love to see pictures of said horse though, and see if it really is what the guy says it is.