Legal Question in Real Estate Law in New York

If you truly are partners (sharing of profits and losses) then the only remedy is an action for an accounting. If that action shows a balance in your favor and against him, you can secure a money judgment. You then can collect that judgment, if need be by selling his share of the house.

But it's not likely that you are really partners. You're more likely to be joint tenants or tenants in common. In that case, you sue for money had and received, on the theory that you paid his share. And again, you sell his share to collect the money. Or he surrenders his share in exchange for being excused from making the payment.