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 mean [and this is what appears from the question] that this person takes from you 90 Riyals as a loan so that he will give you back a prepaid telephone card worth 100 Riyals, then this is not permissible because it is a loan which brings a profit, which is the amount of 10 Riyals on top of your loan.
Ibn Al-Munthir said: “The scholars agreed in a consensus that if a creditor conditions on the debtor an interest [a profit] or a gift, and he lends the money on that basis, then this profit is Riba (interest or usury).”
However, if you mean that you buy from him the prepaid telephone card of 100 Riyals with 90 Riyals cash only, and you will take the card after a month, then this is a permissible sale as long as this is not a way of borrowing money with Riba, otherwise it is forbidden as the Prophet said: “Indeed actions are judged according to their intentions. So every one will be rewarded for what he intends.” [Al-Bukhari and Muslim]. Ibn Rajab when interpreting the above narration said: “The contracts which are initially intended to achieve something forbidden are not valid, like the sale contracts which are meant to achieve Riba and the like; this is the view of the Maaliki and Ahmad Schools of jurisprudence and others, because a person intends Riba by this contract and not the sale, and everyone will be judged according to his intention.”
Furthermore, it is permissible to sell a mobile card with a delayed payment and with more than its price provided the religious requirements are met for selling by instalments; for more benefit, please refer to Fatwa 82986.
However, it is not permissible to sell a salary as this is selling cash with cash, and it is a condition that it should be with equal amount and hand in hand and with the same currency. If the currency differs, it is a condition that the sale is done hand in hand, but this is not the case in the questions which you asked.