When a group of people decide to go Dutch on a trip or a visit to the restaurant, one person may take the responsibility of paying all the expenses for the entire group (usually out of his/her own pocket, but they can also collect budget from the participants in advance) so as not to engage each group member in payment processes and save time. After the event, members of the group compensate by paying their share to this person.
What would this person be called?
If it helps, there is a word for this in Persian, 'Madar-e- Kharj' (literally mother of expenses):
"You don't need to pay money now; John is [the group's / the trip's] Madar-e- Kharj. We will all reimburse him later."
Best Answer
The official answer would be treasurer. From American Heritage:
That said, general usage is only when this is in an official capacity. So you are the treasurer for a club or society, and there may be an official bank account associated with this.
On a ship or a plane, the word purser could also be used:
Both treasurer and purser are somewhat official sounding, and would sound playful at best a group of friends paying for a meal through a delegate. In your example, I might just say "John is paying for now and we'll pay him back later" or "John is handling the money/cash, so pass your cash to him".
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Alternatively, as suggested by @ChrisH in the comments, you might say "John is looking after the kitty". A kitty is defined as (Collins via TFD:
A kitty, though, is usually decided before the meal/drinks/event. It's usually used as an alternative to buying "rounds" of drinks. Everyone contributes a specific amount up front, so the bill is self-limiting. Left-over (or even negative!) kitty can be shared equally among participants.