You can only assume the mortgage if the lender's approval or consent. (At least that's the usual case: almost all commercial mortgages contain a clause or provision in them that there is no assumption at all--though the lender could still consent to it, since any agreement may be changed by mutual consent--or more straightforwardly, no assumption without written consent from the lender, which could be given without requiring an amendment to the agreement.) So it is possible you could do this, but not a given: you need to ask and get the lender's consent.Anyone currently on the mortgage would be responsible for any deficiencies in the shortsale.