I must be missing something, assuming for a moment that slip works both ways (I thought it was only a verb on the outside of doors, but that's a tangent) then let's take it to the extreme version as a thought experiment:
You have a character X, who stays inside a room and never leaves it. They remain there until the end of time and will never be exposed to another character directly.
Character A passes money to X through the door. This money is now effectively removed from the game since X cannot do anything with this money in the room.
Character B later is given money by X back through the same mechanism, and the money now re-enters the game ecosystem through interaction with B who is not inside the room.
Logically, A has effectively passed money to B without having to interact with them face-to-face but no other change to the game has occurred mechanically. However this could also have happened through several other mechanisms, in fact there's quite a lot of ways to transfer money between characters without them interacting in the same physical space.
So I'm still a bit lost how this is different or better than bank transfers, dead-drops, escrow hand-offs, tricksy vehicle mechanics for object transfers, or any of the other existing mechanics.