Is a small amount of money found on the way to the mosque considered a "luqatah" (lost and found item)? Is it unlawful to put it in the donation box without inquiring about its owner? And what should be done thereafter?

A lost item (Luqatah) must be announced for a year if it is of significant value. As for an insignificant lost item that its owner would not bother to look for, it is permissible to take it and benefit from it, or to give it in charity on behalf of its owner, and there is no obligation to announce it. The distinction between significant and insignificant is determined by common practice and the custom of people in the locality.