Deposit gold directly into allies treasury.

[PlebSupporter]

This assumes alliessharedrop is enabled.
Currently dropping a gold pile directly into an allies treasury treats it like dropping onto open ground.

• Considering you cannot pick up gold piles from an allies land, there's no reason to consider the 'mistake' angle as if you accidentally dropped it on their land, well it's their money now. Ask politely for them to return it, and eat shit if it's an AI.
• Yes you can just tell the person where you're going to drop the gold pile and for them to collect it manually, but that does not work for an AI ally, and if said AI ally has a preoccupied workforce, that gold may sit there uncollected for some unknown amount of time.

In which case, it may as well go directly into the allies treasury should that gold land on a treasury tile.