Is it permissible to deduct a portion of the amount upon full payment of a debt that was originally set up in installments, and is it permissible to take a portion as fees without the debtor's knowledge?

Reducing a portion of the agreed-upon amount in exchange for prompt payment (the "give and hasten" principle) is a matter of scholarly disagreement. The majority of scholars prohibit it, while some permit it. However, a reduction made out of leniency (tasamuh) presents no issue. In the case of a reduction, the discounted amount belongs to the purchaser, and it is not permissible for the agent to take it without the principal's permission. If the agent is a broker, it is permissible for him to take a fee or commission, provided the buyer is aware of it.