Sheikh al-Sharîf Hamzah al-Fa`r, professor at Umm al-Qurâ University
If this practice is well known among businessmen and the agencies that send their purchasing representatives know this and accept it, then it is permissible. However, the representatives should not neglect their responsibility to their employers when they accept such commissions.
If, on the other hand, the agencies do not accept this procedure or in cases where the representatives act contrary to the interests of those they represent, then it is unlawful, since it is money that is acquired illegally and deceitfully.
Source: Islam Today