That said, everything in a short sale is negotiable, and there's nothing to prevent the first mortgage holder from accepting your offer but demanding that you pay the second. It's reallly no different from any other offer-counter offer scenario. You can accept the seller's offer, reject it and go away, or make a counter offer.
I have made a lot of assumptions here without knowing the facts of your situation and I certainly agree with all who have said you should consult a real estate attorney. If you don't know of any let me know and I will give you a couple of names.