Buyback—Pay 4 life. (You may pay 4 life in addition to any other costs as you cast this spell. If you do, put this card into your hand as it resolves.) Destroy target nonblack creature. It can't be regenerated.
Official Rulings for Slaughter
10/4/2004 : You can't pay the buyback if you have less than 4 life, even if an effect would keep you from losing for having 0 or less life.