The hit-maker's attorney, Scott Rosenblum, tells TMZ ... his client did collect his money from a $50,000 jackpot on a slot machine Wednesday morning at the Hollywood Casino in St. Charles, Missouri. That's the good news to go with Nelly's bad news -- as we reported, the win led to him getting booked for an active warrant.

Sources familiar with the situation tell TMZ ... Nelly initially thought the arrest was a joke or a prank, as he was in the middle of collecting his winnings when a background check revealed the open warrant.

We're told Nelly was first handcuffed to a chair ... with one hand free to collect the remainder of the cash.

After collecting all of the dough, we're told cops then cuffed both of Nelly's wrists and whisked him away through the casino.

Nelly's lawyer claims an arresting officer told his client they needed to run a background check before giving him the money.

The rapper insists he'd previously won bigger amounts at the same casino without getting a background check. During this background check, an active warrant for a prior traffic stop issue came up ... prompting the arresting officer to search his belongings, where they say they allegedly found Ecstasy.

Nelly's lawyer tells us prosecutors have yet to charge the rapper with drug possession ... and now he wants the arresting officer investigated, claiming Nelly was searched without probable cause.