Andy Drury has become the second key player to leave Crawley in the space of two weeks.
Drury follows Kyle McFadzean out of the door at the Checkatrade.com Stadium after joining former club Luton Town for £100,000.
Boss John Gregory had said he wanted to build his team around the midfielder when rejecting two previous bids for the 30-year-old.
But Reds had no choice but to let Drury return to his old club once Luton offered £100,000 which triggered a release clause in his contract.
Chief executive Michael Dunford said: “When Andy joined us he insisted on a clause in his contract whereby any club prepared to meet our valuation of him would have permission to talk to him. Luton have met that valuation and Andy has decided to go back there.
“The money received will all go back into strengthening the squad. John Gregory and the board are working hard on bringing in further new faces to add to the nine we have already signed this summer.”