Lil Wayne's $51 million lawsuit against Birdman could hinge on what Baby did with $70 million of the $100 million Universal advanced Cash Money for Young Money.

Birdman says he spent the "lost" cash on royalties, marketing and recording expenses. But Wayne’s not buying it, and had asked the judge to force Baby to open his books.

In response, Birdman's lawyers argued that Cash Money isn't much for bookkeeping and the lack of receipts on the missing millions "is what it is."

According to TMZ, the judge didn't like that explanation, and is giving Baby 30 days to come up with a detailed accounting of what happened to the $70 million.