Legal Question in Real Estate Law in California

There is a rebuttable presumption that the cost of ownership is equivalent enough to the rental value that it is a wash. Under that principal, you should not be contributing anything and he owes you nothing. That is really only relevant in a partition action, however, in which one or both of you was raising a claim of unequal capital contributions. During the ownership and before partition, both parties have an equal right to occupancy without payment to the other. Which brings us to your second question, because continued co-ownership with neither having a legal right to force contribution to the ongoing expenses, and with an upside down value, really makes no sense. Failed co-owenerships with negative equity are one of the VERY few situations where a short sale might actually be beneficial to the sellers, but only maybe. A strategic default and foreclosure may still be the best option. There is no way, however, to advise on that complex an issue via the internet. I see by your zip code that you are in Hayward. I would be happy to meet with you, or both of you if you want to approach the debt/ownership issue jointly, in my Oakland conference facility to discuss your options.