When do trading deals become too unbalancing?
As ICodeForCoffee said in the "bribing off the Robber" question:


  According to the rules of Catan, players can't trade unless they're both exchanging resources.


This would seem to be the same issue with the agreement to give extra resources to other players.  They could form an alliance to trade the resources, but not just give the extras "in the future" for a favor.