I don't think that fits with the spirit of the item, and leans back towards the monkey's paw. What you have to do is make it sillier. If they wish for 100gp, they can take out 1sp from their pockets at a time. If they wish for a million gp, it's a random location each time.
The touch of your shadow-wreathed hand can siphon life force from others to heal your wounds. Make a melee spell attack against a creature within your reach. On a hit, the target takes 3d6 necrotic damage, and you regain hit points equal to half the amount of necrotic damage dealt. Until the spell ends, you can make the attack again on each of your turns as an action.
The boss has legendary resistances, so it has a set number of times where if it fails a saving throw to avoid a spell or other effect (such as banishment), it can choose to succeed instead. Divination wizards have the portent ability, which allows them to replace a roll by anyone they can see with a prerolled die from earlier in the day. Legendary resistances make this irrelevant for saving throws, since the boss can just pass regardless of the roll, but initiative to determine turn order is not a saving throw so it can't use legendary resistances.
I see what you did there.