Is keeping the employee's excess change that rightfully belongs to others considered theft, and does this entail a right for the harmed party with Allah, and does this right lapse with forgiveness?

You have the right to take your full due, and the accountant is not permitted to withhold it from you. However, if you leave it for him due to its insignificance and the difficulty of collecting it, then there is no blame upon you or him. You have no right [to claim it] in the Hereafter unless you were deprived of it and it was taken from you without your consent. But if you relinquished it willingly, then your right has been forfeited.