During the first month Jacob has already started working for Laban, and has made himself so useful that Laban realises that he can take advantage of this situation. Jacob is a skilled shepherd and Laban has sheep, so there is an obvious reason to persuade him to stay.
Laban was too happy to accept these terms because he valued the service he was getting out of Jacob. Honest dealing means that we do not hide what we intend to do when we enter into a bargain, but make clear our conditions from the start. No honest person makes an agreement and then changes the terms of the agreement afterwards.