What about the chest he his before he died and became a shadow? Or I like the dead adventurer ideas but that can get old you find yet another dead side with 5 silver and a option
The loot doesn't have to be FROM the creature killed: it could be the loot dropped by past victims, or items that were in the area the incorporeal creature now haunts.
For example, you're in a dungeon previously inhabited by dwarves. The players are making their way through a market district, with a smithy and armorer's to their right, a general store, and a jeweler's to their left. As they approach the area, some number of shadows (either the creatures that killed off the dwarves, or the remnants of the deceased dwarves themselves) emerge and attack the players. After being defeated, the PCs are likely to go investigate these derelict shops, finding some useful oddments, maybe some armor or weapons, some gems and precious metals (possibly in both crafted and raw form), and some coinage in the registers, or on the corpses of the dead proprietors (and even patrons).
As another, the players enter the dormitory of a commander of the city watch, now believed to be haunted by his vengeful spirit. True to the rumors, a wraith emerges from the dust-covered bed and attacks. After eradicating the thing, the players rummage through the ex-commander's dust-covered possessions, finding his old armor and weapons, a half-full coin purse, a ring of Charm Person and a journal noting how stumbling across this beauty of an item turned his career around, and a safe hidden beneath the floorboards containing the man's savings, a deed to a farmstead he was planning on retiring to, and some emergency potions of Cure Light Wounds, Alchemist's Kindness, and Remove Disease.
I wouldn't give any loot.
Not every encounter needs loot. As long as the PCs maintain enough items overall to keep up. I don't use XP, so encounters with utterly no mechanical reward aren't unusual in my campaigns and no-one complains. Usually the PCs discuss how they could have avoided the encounter or what story implications it has, if any.
Well you are gm can just say they have a bag that works that's easy
When they die they disappear leaving behind onyx gems, apparently the gems have a strong connection with negative energy being a component in the animate dead spell and similar spells.
It also makes your local necromancer a happy necromancer.
I like that idea ^
Another idea would simply be to add a similar amount of treasure to one of the party's future encounters. Have them run into a particularly well stocked, lone goblin the next night or whatever happens to come up.
Yea, it's easy when you're in a building or dungeon or something. That's somewhere you'd expect to find various remains from previous victims or items that belonged to the shadow in its previous life.
The issue comes with wilderness encounters. Specifically, wandering monsters in a wilderness. How would you handle loot when the shadows jump the party while they're walking down the road at night?
That could apply in the wilderness as well--the players get attacked by the shadows, and afterwards they stumble across the remains of some unlucky adventurer, or a camp, wiped out by a shadow, which explains why there was more than one attacking them.
Or, just don't give loot for that encounter, then give more for another encounter that makes more sense to you.