I usually sell off work here and there when I reach saturation point. All new work that comes in is priced higher than any outgoing work.
Could you not increase their price instead of selling it?
I keep enough work to keep me going 4 full days per week, I'm not interested in working any more than these 4 days so from time to time I sell off some of the excess work, the work I sell off is the PITA work, I only take on new work in the areas where I'm established, so my work is continually evolving and improving, that never stops.