Unless you WANT to be paying for pony that isn’t yours, that your daughter can’t ride, that probably needs retraining before attracting any buyer… I’d say give the pony back. Or send to trainer of owner’s choosing and let them know they need to resume paying for pony.
What if after a few (or many) months of sinking $$ into pony’s board and training, owner changes their mind and wants pony back? is she stuck on 10k figure?
Seems a lot harder to sell or lease out the pony also, if any agreement has to go through you and then to owner. It may cause potential buyers to find it a strange or messy situation that they don’t want to be involved in. It might be one thing if she were offering you 50% of pony’s sale price; or trying to give you the pony, and you could choose to sell, lease out or give away pony.
It also seems like a gigantic and disproportionate favor or feeling of responsibility for friend who does not seem to reciprocate the concern.
Point out to friend that there is no benefit to you, not even enjoyment of riding the pony, to keep paying for pony, never mind paying even more to get it sold. She can come up with a deal with trainer herself. If it makes you feel better, I suppose you could offer to pay some sort of lease fee for the months you’ve already had the pony?
If owner needs to unload pony asap, and pony is decent-but-needs some training, she could probably find someone to take on pony for free. She can ask trainer(s) if they want a free/cheap pony for their lessons or to take on risk of resell themselves.
There’s some benefit to offsetting training fees with WS/lessons, but trainer and owner need a sense of urgency to sell also, or you’ll likely end up paying for pony for far longer and at greater cost than you would like. And, as mentioned above, it will end up cheaper, and maybe easier also, just to give away pony to someone else now. Heck, it might even be cheaper to pay owner to take back the pony (ie, pay some of the lease fee).