I use a loot-threshold based system for awarding XP in my Dungeon Fantasy game. Old school gold-for-Xp updated to a system that wouldn't deal well with 1:1 like in the Good Old Days.
But I also like to enforce a rule about "new" loot. That is, newly found loot counts towards the threshold, but previously discovered loot does not. The basic idea is to prevent loot-maximizing.
Example: The party solves an interesting puzzle problem, and finds a secure secret door to a treasury. So they take just enough loot get everyone maximum XP. The next session, they do other things that aren't loot heavy, and stop by the treasury and trigger the clever mechanism and take, again, just enough. Repeat until the treasury is empty.
Instead, I only award loot-based XP for treasure when first found and successfully exploited.
What about edge cases?
What if you give loot to a monster and then, much later, take it back?
Probably not loot. It might be if it's different characters, but don't bet the delve on it.
What if you find monsters to trade with?
First time is probably loot - it's a good encounter and exploitation of it.
Second time? Eh. Probably not.
What if you find something, and then only later come back and take some?
Likely it's loot for XP purposes. Discovery isn't exploitation.
What if it's a huge hoard, and coming back and getting more is a dangerous and risk-filled prospect?
It's probably loot.
To me, no edge case has really been that strong of an edge case. They generally seem like a pretty choice in one direction or the other. So that's how I rule and how I play it. I try to point them out ahead of time, if you're planning a delve around a "loot" source . . . but I won't always point it out during the session. I feel like that unduly influences decisions during the session itself and that's not a positive thing.
If you're running loot-based XP, especially if you're doing so using the rules in DF 21, what do you think? How do you handle this?