Someone asked me to buy him a plane ticket.

Answer

It is haraam to charge R2,500. Firstly, a ticket is not material commodity which could be traded in terms of the Shariah. A ticket is merely confirmation of the right to be transported on the plane, etc. Secondly, it will be deception to convey the impression to the person that you had paid R2,500 for the ticket. Thirdly, the R300 gain will be riba acquired by deception. Fourthly, no fee for the service was arranged. A stipulated fee may be charged for a service. It could be arranged to be paid a fixed amount for a service. This amount has to be stipulated prior to undertaking the work.