It seems to me, the issue isn't the hay, but rather the fact that you gave your word to the previous owner who cared enough about the horse to insist on not changing the hay.
I've got to say I admire you for meaning what you say. You gave your word and you stick to it. This is a quality that speaks volumes about your character. I for one applaud you. "Unto thy ownself, be true
". I ask, what is a man, or woman, without honor and integrity?
At some point, you're going to run out of hay. What then? I mean, this does seem a little open ended to me. Maybe you could speak with the previous owner and work something out where you can keep your word to him and the previous owner will also be kept whole in the process?
Try to renegotiate?