Is borrowing five hundred dinars to be repaid as seven hundred dinars after three months, without increasing the amount for delayed payment, considered riba fadl (usury of surplus) or riba nasi'ah (usury of delay)?

Riba al-Fadl (usury of surplus) is the sale of money or food items of the same kind with an increase, and it is forbidden by the Sunnah and scholarly consensus (Ijma'). As for Riba al-Nasi'ah (usury of delay), it is the stipulated increase that the creditor takes from the debtor in exchange for deferment, and it is forbidden by the Qur'an, the Sunnah, and the consensus of the Ummah. The evidence for this is the saying of the Prophet (peace be upon him): "Gold for gold, silver for silver, wheat for wheat, barley for barley, dates for dates, salt for salt — like for like, hand to hand. Whoever gives more or takes more has engaged in riba. The taker and the giver are equally culpable." The scenario mentioned in the question falls under Riba al-Nasi'ah.