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."