I created a magic item for one character and another character took it

I’m DMing for a group of friends, and they rolled really high stats… except for the dragonborn warlock who ended up with a Constitution of 6 (-2). (We did use 4d6 drop lowest… the warlock was just really unlucky.)

Now that all the characters are third level, he only about half the hit points of the other characters. With that in mind, I put into an adventure a magic amulet that increases the wearer’s Constitution modifier to +2.

Before the warlock could find the amulet, though, the party’s rogue took and kept it. I don’t want to kill the rogue’s character over this: His personality trait is ‘I see treasure and I take it’ the only way I’ve found this being role-played is after the battle and looting bodies or the environment, opening crates barrels etc. (It’s possible this is the player being a jerk, but I don’t know if I should hold the player accountable for his “character’s” actions.)

The rogue also sometimes asks—demands, really—extra XP for killing the first monster or for killing the most monsters, and mostly I say no. When I do say yes, it’s a small amount like 25 XP. The rogue has plenty of treasure, by the way, including his own unique magic item.

When I told the rogue’s player that the amulet was for the dragonborn, he said, “Oh, well! Finders keepers!”

I’m thinking of a having a monster or NPC just take the amulet, or maybe offering to trade the amulet for another seemingly more powerful (but actually cursed) magic item.

How should I continue?