Sounds like more of the same…
If PO can’t take back the pony and can’t afford pony anymore, seems like the easiest thing to do would be to sign pony over to you for free (so you can sell/give away pony free and clear, not that I’d want the pony at this point), or to a trainer willing to take on the pony and (re)train & sell or use her as a lesson pony.
PO wants the reward (sale money) with none of the risk (paying for pony until she sells), and that’s just unrealistic and unreasonable. At this point, even if you find a good trainer and have a decent agreement/training & sales contract with them, the issue is still going to be between you and PO. Who’s to say she won’t change your (spoken/unwritten) terms again? Or hold out for a high, unattainable sales price?
If you’ve given notice and PO doesn’t pay board, then BO can put lien on pony? And then pony is their problem. Unfortunate for BO, though.
If PO is using you as an agent of sorts to get pony sold… I’d say get a contract in place for that too. I’ve seen “free leases” where the leasee is paying bills on the horse, until horse is sold, and sales price goes to owner… but everyone is in agreement on this and generally the rider/leasee is getting something out of it, like a nicer horse than they could usually afford, even if it’s for a non-specified time period because horse is for sale during lease. Doesn’t sound like the case here!