How can interest on paper currency be considered explicit usury, knowing that its value is variable and not linked to gold or silver, and that the value of the interest along with the principal may change amidst fluctuations in the value of paper currency, while prohibited usury is an increase on a fixed principal by a known increment?

Usury (riba) in currencies is forbidden by Islamic law. Its prohibition is not waived by changes in their value, whether rising or falling. It is not permissible to use the clear change in currency value that occurs during disasters and wars as a basis for permitting usury. Whoever seeks profit should invest their money in a permissible investment, as there are many legitimate avenues for investment, and they are not limited to interest-bearing loans. Allah has permitted trade and forbidden usury. Many scholars have considered cases of harm due to currency value changes and collapse in relation to debts.