All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If you are an agent acting on behalf of your brother in this transaction (it appears that you are so) then it is not permissible for you to add an additional amount to the price with which you will buy the computers. You are permitted to ask your brother to give you a commission for acting as an agent in buying the computers for him. However, it is not permissible for you to inform him that the price of the computers is such and such and the reality is otherwise, as this is consuming money without any right and this is something which Allaah forbade. Allaah Says (what means): {And do not consume one another's wealth unjustly.}[Quran 2:188].