The BO is trying to maneuver it so that she has a financial interest in the transaction. Add that to the pressuring, and the whole situation starts to stink.
The BO will be getting paid by the OP while her horse does nothing. So I am missing your point, honestly.
It is hard for me to believe that this BO is that evil and horrible and is running off on their own plotting behind the OPs back when the OP is willing to leave her beloved horse in this person care.
I would guess the BO thinks they are helping and it sounds like the OP is afraid to just tell the BO this is not what is wanted anymore.