Supply and demand!
If he wants you to sell them to him ... sell them to him.
At a profit.
If the domains are worth more to you than what he will pay ... keep them.
Obviously, there may be more to the question ... like future relationships with the client ... past relationships with the client ... and the client's power to influence other would-be clients in the future.
But with the info you've provided ... I say, Make Him Pay!!
Robert