Is it permissible to pardon a deficit or surplus when exchanging metallic currency for paper currency of the same type, within the same session of the transaction?

Some scholars permit disparity when selling paper currency for metal currency. In this case, there is no objection to the aforementioned transaction because equality between the two exchanged items is not stipulated. However, the preponderant and authoritative opinion is prohibition, as it stipulates equality between the two exchanged items. Ignorance of equality is considered equivalent to knowledge of disparity, which makes the transaction a forbidden usury of increase (riba al-fadl). The safer course of action is to refrain from it.