Excess bandwidth trading

We have two Office::1 accounts, one which has little usage and one which frequently goes to the wire. It'd be nice to trade some of the excess bandwidth between the two accounts.

As a wider idea, if someone has bought too many units and someone else has bought too few, being able to gift the extra would be good. AA still get the same money (well perhaps a bit less from topups) but it means you can do things like help someone who turns up at the end of a month on IRC worried they're about to run out.