Is it permissible for a sports coach to buy an offer consisting of several types of proteins at a reduced price, then sell two of them to trainees at the agreed-upon price, and keep the third type for himself, knowing that the total price of the offer was 120 dinars, and he sold the two types for 100 dinars, and kept a type worth 20 dinars?

If the trainees are dealing with the trainer as their agent to purchase the mentioned items, then it is not permissible for him to take the price difference. The three types [of items] belong to the trainees, and the trainer has no right to dispose of them without their permission. However, if the trainees are dealing with the trainer as a merchant who buys for himself and then sells to them, there is no harm for him in selling them at the agreed-upon price.