Is it permissible to store mobile phone top-up cards and not sell them now with the intention of their price increasing, or is that considered monopolization?

Scholars have differed concerning the scope of prohibited hoarding. Some have limited it to foodstuffs, while others have extended it to include everything that people need and are harmed by its withholding. This latter view is the correct one, consistent with the apparent meaning of the ahadith and the wisdom behind prohibiting hoarding, which is to avert harm from people. Accordingly, if storing cards does not harm people because an alternative exists, then there is no objection to it and it is not considered hoarding. However, if not selling them harms people and causes them distress due to the lack of an alternative, then it is prohibited hoarding.