What is the ruling on a sales representative taking company gifts designated for customers for himself, or for his family and friends, whether with or without his manager's permission, given his prior approval?

The gifts given to you to be handed out to customers are a trust (amanah) that you are not permitted to dispose of in any way other than what you have been authorized to do. Taking any of them for yourself or your family requires explicit or customary permission. If your knowledge of your manager's approval is considered customary permission, then customary permission takes the place of explicit permission. However, you must ascertain whether your manager is authorized to give permission in this regard. If he is, then there is no harm upon you; otherwise, his permission is not valid.