What is the ruling on selling phone credit at a price higher than its original value using a "guarantee card" that is cashed at a later time? And is it permissible to reduce its price for relatives? And what is the ruling on buying this card in cash at a price lower than its original value?

It is not permissible to sell cards that are considered a debt for deferred cash, because that is usury of delay (riba al-nasi'ah). It is permissible to purchase communication credit with these cards, even if the price is more than the current value of the credit. The seller is not obligated to unify the price for the general public; he may reduce the price for whomever he wishes and raise it for whomever he wishes. What is forbidden is only deception, fraud, and misleading the ignorant. There is no limit to profit in selling as long as there is mutual consent between the two parties, unless the buyer is ignorant of prices, in which case the seller is not allowed to deceive him.