What is the reason for the requirement of immediate possession (al-taqabud) when exchanging currencies?

The origin of the obligation for immediate possession (al-taqabud) at the same sitting when exchanging different types of currencies is the hadith: "If these types differ, then sell however you wish, provided it is hand to hand." Possession was stipulated in currency exchange for the purpose of designation, as dirhams and dinars are designated by possession. If they were to separate before possession, it would become a debt, which is the prohibited sale of debt for debt (بيع الكالئ بالكالئ).