How is a loan amount, given in euros in 2021 and converted to Egyptian pounds at the time of lending due to banking restrictions, to be repaid, considering that the value of the euro has doubled against the pound since then, and no prior agreement was made on the method of repayment?

The ruling on this matter depends on the initial agreement between you. If the borrower requested the loan in euros, and you could only convert it to Egyptian pounds, so he instructed you to convert it to pounds for the transaction to proceed, then the agreed-upon loan currency is the euro, and the debt is established in his liability in that currency. However, if he requested the loan in Egyptian pounds, and you converted euros to pounds to give him the amount he requested, then the debt is established in the currency he actually received, which is the Egyptian pound.