Is it permissible to repay a loan with interest after having agreed to it out of ignorance, and is that considered usury, given the desire to avoid the lender's anger and adhere to Islamic law?

It is not permissible to give the requested increase, as it is forbidden usury (riba), and Allah has threatened those who commit it with severe punishment. You must repent to Allah for agreeing to usury. Allah the Almighty said: "O you who have believed, fear Allah and give up what remains [due to you] of riba, if you should be believers. And if you do not, then be informed of a war [against you] from Allah and His Messenger." The one who consumes usury, its giver, its scribe, and its two witnesses have been cursed. Scholars have unanimously agreed that stipulating an increase in a loan is usury. Explain this to him; if he accepts and repents, that is what is desired. If he becomes angry, it does no harm, for pleasing Allah is more worthy than pleasing people.