Someone who is working for Jewish Owner took a lot of time for his personal things to take care of. There is no way to make a calculation of how many hours it was, so therefore there is no way to return back the money taken in payroll. Also, there is no way he can ask the owner Mechila because he might lose his job. Is there a Derech a Teshuva in this type of case.
ב"ה Since this question, has to do with monetary issues (= דיני ממונות) I consulted with another Rabbi form our website, Rabbi Moshe Meir Aviner, who said the following. First of all, you should pray to Hashem to assist you to do Teshuva. If it is possible, you should add work hours without receiving payment. After a period, in which you assume you have possibly completed the hours which you used for yourself, you should approach the employer and ask his forgiveness. Since, he will now have seen your exerted effort and that you have changed your ways, and he will probably forgive you with a full heart. In the book "Orot Hathshuva,( Chapter 13:9) Rav Kook zt"l writes "A person should not be frightened by the obstacles on the path to Teshuva. Even if Teshuva is difficult, because of things "Ben Adam Lechavero" between fellow men, even if he knows that he is not fulfilling his full obligation of Teshuvah, and because of different weaknesses he cannot rectify the things "Ben Adam Lechavero", a person should not leave in his heart any weakness of mind which may cause him to minimize the value of Teshuva. And there is no doubt that stemming from complete Teshuva for things for which he has nothing holding him back, Hashem will help him to rectify all the things for which he doesn have things holding him back which he cannot overcome. All the Best