Is taking Qur'ans from a mosque in another country considered theft, and what should be done to return them given the difficulty of going back, and is it permissible to donate many times their value as expiation while keeping them?

It is obligatory to return the Mushaf (copy of the Quran) that was taken from the mosque, because it violates the condition set by the endower (waqif). If it is not possible to return it to the same mosque from which it was taken, then it should be placed in another mosque.