If you let the employee take PTO he or she did not earn subject to the explicit agreement--i.e., he or she had agreed at the time--that if he or she left before earning out the PTO he or she would be liable for the cost of the time, you could sue the employee to recover the money. However, without an explicit agreement to this effect, you will not be able to sue. Even with an agreement, a lawsuit is the only way to recover it, which is far from optimal.In the future, it is best to not let employees use PTO they have not yet earned.