Security deposit as final month's rent

[Garrett H.]

Any insight on using a tenant's security deposit as their final month's rent?

After giving notice that he and his family are moving out of the building, a tenant asked if he could surrender his security deposit to cover the final month's rent.  Not an ideal situation but I'm willing to go along with this scenario.  Given the famously contentious air surrounding the matter of security deposits here in Chicago, I asked if he were willing to sign a form agreeing to this arrangement.  I was considering drafting something up but I'm wondering if there is a more formal document out there that could do the trick.  Anyone have experience with this sort of situation? Thanks in advance.