Using words such as "Sadaqa Allah-ul-Azhim" to conclude the Qur’an recitation with is a Bid‘ah. It was not authentically reported that the Prophet (peace be upon him) ever said them after he finished reciting the Qur’an. Had it been lawful to conclude the recitation with these words, the Prophet (peace be upon him) would have said them at the end of recitation. The Prophet (peace be upon him) was authentically reported to have said:
Anyone who introduces anything into this matter of ours (Islam) that is not part of it will have it rejected.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.