Olivier Thomert has joined Rennes from Lens, with Olivier Monterrubio moving in the opposite direction.
Thomert, who was widely linked with a move to Manchester City last summer, has put pen-to-paper on a three-and-a-half year deal with Rennes.
The striker is looking forward to the new challenge, but is puzzled as to why Lens suddenly decided to let him leave.
"Indeed it has been a surprise, even a little for me," he admitted.
"A few days ago I met Lens officials and they told they wanted to keep me, but then things changed.
"I believe Lens wanted to get some cash, but I don't understand everything.
"I met the Rennes coach and chairman in Paris on Wednesday, and it has been very nice."
Monterrubio, meanwhile has agreed a two-and-a-half year deal with Lens.