And they are really going to agree to that if I just ask?
Happens all the time,especially if you live in a non-recourse state.
If you are seriously contemplating /needing to do this consult a Realtor who has the CDPE qualification.( Go here) CDPE
Be aware that the shortfall between what you sell your house for and the mortgage outstanding is considered by the IRS as " Forgiveness of Indebtedness Income" and normally considered income for tax purposes. Congress has waived that provision of the code until the end of 2013, it may or may not renew that in the new budget - if and when we get one.