Please contact your current mortgage lender to see if it is even legal to do a lease purchase of your home to a new buyer. If you are not allowed and proceed with a wrap mortgage or lease purchase and your lender finds out they can call your note to be paid in full immediately and you could lose the house to foreclosure if you can't pay off the note.
The first thing you'll need to know is what the most likely price your home will sell for. Get a quality market analysis of your home so you'll know the true numbers you're working with. If in fact, you're really 20K upside down, you will have few options. Depending on the price range of your home ($100K vs. $400K), you may be able to do some cost effective improvements to give your home greater value without hammering your checkbook. If so, that could reduce your current negative equity situation at least a little. You may also consider selling by owner so commissions won't be factored into the equation, but get some good advice if you choose this route. Finally, you mght consider owner financing for the new buyer. Buyers who are unable to obtain an institutional loan recognize they will have to pay more for a home that is owner financed. Just do a 3 or 5 year balloon note so you can get out of the house when the market turns. Best of luck.