alloprophoric

"bringing on behalf of others" but implies the selfish twist: the person is bringing something that isn't their own to benefit themselves.
The noun for this would be an allophorist, or the act itself could be called allophory.

You host a BBQ at your house and your brother-in-law makes a ridiculous amount of plates for his friends to come and pick up

"Yo! Stop this alloprophoric behavior at once!"