This is the way the scam works:
A person claiming to be a potential roommate sends a check for an amount far above the amount owed (this is a HUGE red flag). They then request that the balance be sent back to them so that they can buy an airline ticket or some other expense (which is bogus). The check they send is fake.
Check the links and instructions I provide in this post.
Also, go to FakeChecks.org and report it.