What's the ruling?

Team A has the Ball on B's 5 yard line. The ball is snapped and the Quarterback goes back to pass. The wide receiver runs a down and out pattern in the end zone and inadvertently steps out of bounds (he was never forced out of bounds by the defense) The receiver realizing he stepped out of bounds returns in the end zone. The quarterback spots the same receiver in the end zone and throws him a completed pass. What's the call?

I have Illegal participation enforced 15 yards from the previous spot since it was during loose-ball play. Replay the down from the 20.

Sounds good to me.

OK guys What are your responses to the following Reference. In the case book under "illegal Participation" in the comment section on top of page 70 it states.....
"When A1 goes out of bounds voluntarily or by accident, or delays his return after being blocked out by an opponent, he commits an illegal participation foul when he returns inbounds. The spot of the foul is were he returns inbounds. What's the call now?

Same call as before?

The spot of the foul is in the end zone. However, the basic spot during this loose ball play is the previous line of scrimmage. As the spot of the foul is not behind the basic spot, we'll enforce from the basic spot. Replay from the 20.

No player of A or K may go OOB a return to participate.