Is what the questioner did correct by leaving 10 Egyptian pounds in the savings account and considering them a completion of the amount of usurious interest that he donated to the mosque, due to his inability to withdraw the full amount?

A savings fund that offers a fixed percentage on money is impermissible to subscribe to because it is usury (riba). Whoever repents from usury is entitled only to their principal capital, as Allah Almighty says: "And if you repent, you will have your principal — [thus] you do not wrong and you are not wronged." (Quran 2:279) Whoever has taken usurious interest must dispose of it by spending it on charitable causes. If the fund stipulates that a certain amount of money must remain with it, then there is no blame upon you for leaving a sum of money in it, because you are compelled. However, you have no right to this amount, and you may not demand it from the fund from now on.