Royce “Iron Ron” Corley was arrested for selling underage girls in Manhattan — but the borough’s district attorney was powerless to lock him up for trafficking.

Instead, he had to turn to federal prosecutors to make the case.

The 29-year-old Corley had taken in three 16-year-old runaways — giving them lodging and cellphones — and then advertised them for sex on Backpage.com, posting explicit photos of the teens in the classified ads.

The Manhattan District Attorney’s Office and NYPD busted Corley in an undercover operation in 2011 but didn’t have what was needed to prove the girls were forced, coerced or defrauded into prostitution under current state law.

Federal laws recognize that underage teens can’t consent to sex, so DA Cyrus Vance Jr. handed the case off to then-US Attorney Preet Bharara.

Corley was sentenced to 10 years in prison for sex trafficking minors and possessing child pornography.

But prosecutors say it’s no long-term solution — they can’t force the feds to take a case, and it means the attorneys who know the case best are no longer involved.

“It just drives you crazy,” says Vance of the antiquated law.