What's the Penalty?

[varefump]

NFHS Rules.

Team is using a Libero and the Libero has already served once during the game.

The Libero is in left back and the team is awarded a point/loss of rally.
The legally replaced player comes in for the Libero and the right front leaves the game with the Libero going back to serve.

However, the Libero did not serve in this position in the line up previously.

Should it be a violation at this point since the Libero is not allowed
to serve in this position, thus making it an improper replacement,
or does the violation not occur until the Libero/server contacts the ball for serve as the wrong server.

Is the penalty Improper Replacement (unnecessary delay) or wrong server (point/loss of rally)?

Thanks in advance for your response.