What should he do with the leftover donations?

What he must do is ask permission from the donors to dispose of it as he wants on permissible or recommended things. It is not permissible for him to collect charity or donations in the name of a specific cause then spend it on something else, unless he does not know the identities of the donors and has no means of contacting them. In that case he must spend that charity on the closest cause to that for which he collected it. So if a person collects money to build a mosque, then there is extra money, he should spend it on building another mosque, and he should not spend it on the poor and needy, for example. 

This is what the brother who asked this question should do. If it is possible for him to tell the one who donated a large amount about the situation, and ask his permission to dispose of it, then he should do that. Otherwise he may spend it on the web site and buy what is needed for it. 

The scholars of the Standing Committee for Issuing Fatwas were asked: 

I collected donations to build a mosque in the village, and the mosque was built. Some good people donated money to complete the interior, because the money that I had left was not sufficient for that. I have some of the donations left. Is it acceptable to spend the leftover donations on building fences for the Eid prayer place and graveyard in the village? Is it acceptable to spend the extra funds on buying air conditioners or furnishings for mosques in other villages? Is it acceptable to spend the extra funds on helping those who want to get married or on the poor and needy in the village? All of that is with the intention of the reward going to the donors. Please note that the donors include both villagers and outsiders. 

They replied: 

It is permissible for you to spend the remaining money that you have left over from building the mosque that you supervised on building another mosque, if the first mosque does not need that. It is not permissible for you to spend the money mentioned on anything other than building or maintaining mosques, because that is spending it on something other than the purpose for which it was donated. End quote. 

Fataawa al-Lajnah al-Daa’imah (al-Majmoo’ah al-Thaaniyah, 5/208). 

And Allaah knows best.