I bought Clem for the price we got her at with the condition we keep her at the stable.
Two months later I wanted to move her closer to school, so I asked and he said yes. That's about the option here -just ask, explain you options. If you're told no... Well, it's in the contract that you signed, so really unless you sell the horse/buy a new one you might be out of luck
Otherwise ask for a time limit. Keep it there FOREVER? Or just a year? Or how long? I'd sit down with the BO and talk about it.