As Luthen lands on Aldhani and sets up Andor's personal terms for the heist, he hesitates for a moment. He then takes the sky khyber crystal from around his neck, obviously something of great personal importance to him, and gives it to andor as a "down payment" for the mission, but expects it back.

On face value, this is fucking insane.

Andor is pretty much locked into the mission, what else can he really do at this point? Run from Vel and the team and go where?

On the other hand, this mission is probably going to fail. Luthen has effectively flushed this crystal down the toilet.

Yet.... think about the mission with and without the crystal?

With the fight with Skeen over the crystal, he ultimately heals the deep mistrust surrounding the circumstances involving Andor joining the mission. It provides some catharsis to cut the swelling tension and anxiety the night before the mission.

Nemik flatlines. You hear a shot outside the hospital tent. As you draw the sheet over his head Andor marches in with a gun pointed at you.

Imagine what other item Andor can offer to you in his other hand that in one gesture proves he's not robbing you, Skeen probably did betray you, and Andor is a man of honor and not a scumbag.

If Andor is what you think he is.... why wouldn't he take it?

As far as Andor and Luthen's future goes, if Luthen is ever standing in a large room where the topic of discussion is "Did Andor betray us", he will always have a reminder hanging around his neck.