Is it better to forgive the debtor or to take the surplus of the "double" after the debt period ends, and does this apply if no repayment period has been specified?

The Sharia evidence commands granting respite to the insolvent debtor and considers this a daily charity for the creditor, equivalent to the amount of the debt, until the due date. If the creditor grants respite after the due date, it is considered a charity equivalent to twice the debt. However, waiving all or part of the debt is better than granting respite, because it encompasses both respite and an additional benefit. This is indicated by the noble verse: (And that you remit [it as] charity is better for you, if you only knew) (Al-Baqarah/280). As for a debt for which no due date has been set, the default is that it is immediately due, and the creditor has the right to demand it at once. Every day that passes is considered a charity equivalent to the debt, and in this case too, exoneration (waiving the debt) is better than granting respite.