In California, there's a lease with three people on it (call them X, Y, and Z) and the landlord (L), with six months remaining on the lease. Y, Z, and L agree to release X from the lease. Y and Z do this by signing a form that says the following:
Unless otherwise stated, all terms and conditions of the prior lease... shall remain true and in effect.
By signing this Extension, effective of [insert date] X relinquishes all claim of possession on the Premises and of any held security deposit. The landlord, Y, and Z agree to relinquish X from any obligation regarding the lease as mentioned above. Y, Z, and the landlord remain otherwise bound by all provisions as embodied in the lease as mentioned above.
Now, suppose there were prior electronic conversations in which X unequivocally agreed to pay for the next six months of "X's part of rent" (even when he's not on the lease) in exchange for Y and Z (landlord is not part of the conversation) agreeing to remove X from the lease.
And then X reneged on that offer by refusing to pay once the modification document (the last two paragraphs) was signed. Can Y and Z sue X in small claims for the "unpaid X's part of rent" after the [insert date]? And can Y and Z sue X in small claims before the [insert date] for what's effectively a reneged future rent?