With all the heat on Dexter hed never agree to work for the police under seemingly normal circumstances. It would have to be an explicit agreement that Wallace knows who Dexter is but wants him to work for her rather than capture him. But if that happens, I could see something like this coming true. It would also require good faith on Wallaces part; which means she may ultimately be the cause of Batistas downfall. Imagine Wallace turning on Batista and then pressuring Dexter to work for her to help catch the Ripper. Now, Dexter has little choice but also knows he can somewhat trust her, if she took down Batista and is making the offer.