Or did you get the house and plan to stay there? If you plan to stay and he is not, I think you are stuck with the upgrades. If you are selling (which takes a long time these days), you can offer the upgrade as a credit to the buyers to be taken out of the sales price (you would not do any work until after you have a buyer). You just can't go spending someone else's money and expect them to pay for half.
I hope this helps. There are many legal questions involved so you should probably ask your attorney.