Is it permissible to lend ninety Dinar and stipulate repayment of one hundred Dinar, provided the weight is identical and the currency is only accepted by weight in a specific location?
Chapter on Loan (Qard)
Al-Mughni
Book of Sales
Primary text
This transaction is permissible. Since the currency is only accepted by weight, the excess in count (ten extra units) does not constitute an unallowable increase (Riba) because the ninety units already cover the principal loan of ninety units.
Supporting text
If the currency is accepted by face value in a location, then it is not permissible. This is because when accepted by face value, the extra ten units represent an impermissible surplus, as receiving one hundred for ninety constitutes an unpermitted increase (Riba).