Trying Something New

My DW and I are dogless for the first time since 1972. Cape is retiring in June; and we plan to get another Airedale Terrier soon thereafter (#5). In the mean time, we've become pet sitters. We're keeping a 2 year old Black Lab until mid May. This is her first night with us, and all is well so far. She's a sweetheart as most Labs are. My fear is that, after several months of caring for her, we're going to get very attached to her and vice versa. It may be very painful to give her back to her owner, and it might be painful for Mallie to leave us. (Our daughters tell us that when they are reincarnated, they want to come back as our pets). Anybody ever done this? How'd you handle it?