Soulja Boy -- Armed Robber Wanted His Cash ... Shot His Boy Instead

The armed robber who put a bullet in one of Soulja Boy's friends Friday was actually there for Soulja himself ... sources are telling TMZ.

We broke the story ... a bday guest of Soulja got shot in the arm outside of a recording studio in Hollywood. The victim is Killa J -- one of Soulja's artists. Sources connected to Soulja tell us he now suspects it was all an inside job.

We're told Soulja and his entourage noticed a black Ford SUV with four men in it waiting outside as they showed up to the studio to celebrate his 26th birthday ... and thought nothing of it at first.

As they wrapped up the party ... we're told Soulja's crew checked security cameras and saw the vehicle was still there. Killa J stepped out first to see what the deal was, and that's when four men rolled up and demanded Killa J give up Soulja Boy's Gucci bag, which is usually where he carries large amounts of cash.

We're told they asked for Soulja by name too. Probably not coincidentally ... he'd been flashing his bday stacks on social media during the party.

According to our sources, Killa J refused to cooperate ... and that's when bullets started flying and the Memphis rapper caught one in the arm. He's expected to recover.