Usher's Ex-Wife Tameka Foster -- Don't Leave Me Homeless!

Usher's ex-wife Tameka Foster will be out on the street in less than 60 days -- if Usher wants it that way, and she's totally freaked out.

Sources tell us, the couple -- who's trying to settle a nasty custody dispute -- is deadlocked in mediation, and the biggest bone of contention is Usher's Georgia mansion.

Tameka's currently living there -- but under the divorce settlement Usher can sell the property whenever he wants ... as long as he gives her a 60-day heads up.

We're told Tameka fears Usher is going to pull the trigger -- leaving her buried in legal debt with no place to live -- so she's asking him for a permanent home for her and Usher's kids.

Usher's response ... no way, no how. He says he's already paid her enough.