If owner is making all final decisions, wouldn’t they be the one to pay for pony’s board/training until it sells? Or, rather, why aren’t they the ones paying for pony’s board/training – I mean, you don’t have to answer here, but it’s certainly something I would think about!
I think it’s very nice of you to help out, but generally, lease-to-own is a risk that the owner takes on, that leasee won’t buy pony in the end and they’ll end up with it again. For the amount of time you’ve had the pony, they haven’t had to pay for its upkeep and have made some $$ off his lease fee to you. And if they choose $$trainer, are you stuck paying that cost?
Anyway, if you (and pony owner? whatever arrangement you have with them seems more fraught/one-sided than potential agreement with any trainer, at this point) want to get out from under the pony, I’d take the 5k, if it’s a real offer. Otherwise you could be putting that much or more into it in the next few months to get it re-trained and sold. And no guarantees it will get 10k or stay sound or any number of other things horses can do.
If he needs a lot of work, both to be retrained and to maintain his rideability, then IMO trading training fees for good quality lesson riders (mini-pony-pros essentially) is not so bad. as long as trainer is still working very much on selling him.