Is it necessary to inform the employer about using his laptop bag for another company, knowing that it will not be damaged and that the questioner wishes to repent?

If the employer does not allow the use of the bag except with his own computer, then it is not permissible to use it, based on the Prophet’s (peace be upon him) saying: “It is not permissible to take a person’s property without his willing consent.” In this case, it is appropriate to approve of this restriction. However, if he allows it, or if custom dictates leniency, then there is no harm, because “custom is a ruling arbiter.”