Is it permissible to take a portion of the money that was given to purchase an item after the purchase is complete?

You are an agent for someone who has given you money to purchase a specific item, and the agency can be with or without compensation. If the agency is without compensation, you are not permitted to take anything from the amount unless the owner willingly permits it, because what is in the agent's possession is a trust that must be returned to its owner. However, if the agency is with compensation, you are entitled to claim the agreed-upon compensation. If you typically work for compensation and no specific compensation was agreed upon, then you are entitled to the customary wage.