Ignore this suggestion.

A player might want to give some "in-game" cash to another player while in the same vehicle.
An example of how to give cash to someone while in the same vehicle could be like pressing Y ---> Select player's name (he/she must be in your vehicle)
---> amount of cash that the sender wants to transfer (from wallet or bank) ---> pressing the button "Enter" or clicking on the bar option "Send cash" and the player in your vehicle has received your money.

I’d have to test this but can’t you do this already? I know that’s how you do it on foot.

You can already do that. Just press Y near a player or while being in a car and you can interact with him.