Justin Bieber -- Trapped in L.A. ... With $85,000 Bill

He's already packed up the moving trucks ... he's pretty much hammered out his egging case ... but Justin Bieber still has one enormous loose end to tie up before he can bail on L.A. -- a bill for $85,000.

As we reported, Khloe Kardashian has purchased Bieber's Calabasas mansion, paving the way for his big move to ATL. The deal is expected to close next week, but there's one big problem -- TMZ has learned, there's a mechanics lien against the home.

A construction company filed the lien, claiming it performed $85,011 worth of damage repair services on Justin's house and it still hasn't been paid -- which is a big problem ... because a home cannot be sold until all liens against it have been settled or paid.

If Bieber doesn't cough up the money by next week, we're told the sale can't go through.

Sources close to Bieber tell us, the singer's camp plans to deal with the issue asap. Luckily, he can probably afford it.