Cash for keys is very common, and I agree with the others. Get it in writing, and make sure you understand the agreement (particularly regarding what you can or cannot take from the property).
If you're a tenant, take the money... and find another place. You'll probably come out more or less ahead of the game.
If you're the owner, this is a question of time... if you have received a "Lis Pendens" the clock is running, and you may still have enough time to complete a short sale. The short sale will reduce the damage to your credit, and allow you to requalify for a mortgage in fewer years. Give me a call, and I'd be happy to help you figure out what the best course of action is for you.