It's not like Sally was some kind of powerful adventurer. If he took her with him she would just become someone to be protected at all times, since he's going to fight more and more powerful enemies. And, because of his appraisal, any of her knowledge isn't all that useful either. It's much better for Sally to remain in that town and lead her own life as a receptionist than become an add-on to the MC. He also can't remain there forever, since that area is too low-level for him. But hey, maybe he will return there later.