What is the ruling on the work of a brother who sells goods in installments with an annual increase of 30% while taking guarantees and checks, and also carries out what is called "cash conversion" by selling communication cards at a deferred price to obtain money in cash? Are these transactions permissible or usurious, and what is the ruling on obtaining money in this manner for personal spending or to start a project?

If the goods are present and specified, ownership of them is required before selling them on a cost-plus basis (murabaha). The murabaha sale to a purchasing agent (al-amir bi al-shira') proceeds through three stages: a promise to purchase, then the seller buys the goods, then he sells them to the purchasing agent. There is no harm in taking guarantees such as a guarantor (kafeel) and checks as a down payment, provided that the signing of the checks does not constitute the sales contract before the seller takes possession of the goods.

However, if the goods are described with a deferred delivery (bay' al-salam), it is required to pay the full price at the time of the contract, otherwise, it is not valid.

Selling pre-paid balance recharge cards is permissible at their value or more, whether spot or deferred. This falls under the sale of usufructs (manafi'), and it is permissible to sell them in installments. This is called "tawarruq," and it is permissible.