[... and gaining/regaining loot gives you XP, before you fret]
I had a thought.
In Swords & Wizardry, player characters get XP at the end of the adventure equal to how many GP worth of treasure they manage to haul out of the dungeon environment safely.
It occurred to me that, if we're already going with the premise that there's some link between getting the loot and growing more powerful, then maybe being parted from your loot should make you *less* powerful.
It might be cool to give p.c.'s the option of preemptively relinquishing their claim on a bit of treasure in order to avoid being XP-penalized for it - basically, if they give it away, bury it, throw it away, spend it frivolously, etc., the important thing being that they don't really benefit from it changing hands, they can never lose XP from someone making off with it.
If loot is spent on things that are useful to the player, or the p.c., even if it's something like "I tell our informant that the next round of drinks is on us. That should loosen his tongue!", then you've spent it and there's no need to worry about losing it at all. However, if it's spent on something that sticks around (possessions, property, etc.), and something should happen to those tangible investments, you lose XP equal to their value until/unless you get them back.
This could create some interesting dynamics. Certainly, there's something of a precedent for exploring in this direction: the AD&D paladin has to tithe 10% of his earnings to his church or temple, and some editions and settings suggest or demand that clerics do something similar.
This could set up a scenario in which players dutifully haul their gold back to the front door of the dungeon, and then decide how much to keep for necessities and things, and how much to fritter away. Maybe charitable acts, done outside the context of a "you scratch my back..." exchange, could also count as throwing the cash away. If there's no explicit tit-for-tat going on, such that the recipient of your charity is not literally obligated to help you in return (but maybe the GM remembers your good deed for another day), then you've gotten rid of the gold successfully.
I think this could make things interesting at higher levels, too - the ubiquitous lordly title that fighters earn at 9th level comes with a keep and followers; what if squandering these gifts made you lose XP? Or is that hitting the player twice? Since repairing the damage done bumps you back up to where you were, it shouldn't be too harsh, but it does give me pause for thought about the ramifications.