My first thought is to offer to have a pest control company come out to confirm the presence of fleas, and treat if necessary. From there, I would handle it on a case by case basis depending on the circumstances. If they won’t allow the treatment, they just want out, my gut says they are just trying to get out of lease and not pay penalties so I would charge a lease break. But like someone said earlier, a bad review might not be worth it. If they haven’t moved in, there really won’t be much turn around time and if you’re in a good market, you have a rent ready unit to market. If you think you can rent immediately, just let them out with rent paid and don’t bother with the hassle of bad residents.