"...It has been said." His voice is wry, but his expression has softened.

If you offered to make it permanent, the fool would offer his soul in a heartbeat. Asmodeus sounds - well, as snide as you would expect, confronted with selflessness. Which you really ought to be happier about.

He could make thinks horribly unpleasant for both of them so, so quickly.

But all he says is, For now...well, this should have been part of the deal to start with: he must obey you, for the length of the year.

And fair is fair: he can't attack any devils, through this year or the next.