All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
If that Christian man builds a shop that belongs to the mosque so that its profit will be dedicated to the maintenance of the mosque and to the salary of the Imaam, then this is considered a Waqf (an endowment) and this Waqf is permissible from a Christian according to the view of the majority of the scholars .
The Fiqh Encyclopedia reads: “The jurists are of the view that it is not a condition that the person who makes an endowment be a Muslim; indeed, an endowment is acceptable from a Thimmi (a non-Muslim living in an Islamic state), because an endowment is not a matter of worship in a way that it is not valid from a non-Muslim in principle; rather, getting closer to Allaah with an endowment depends on the intention of getting closer to Allaah by making it; so this endowment without this intention is permissible and thus it is accepted from a non-Muslim just like him emancipating a slave; this is according to the agreement of the scholars, but the jurists disagreed with regard to the items that are valid to be endowed from a Thimmi and those that are not.”
The Fiqh Encyclopedia also reads: “The jurists had different opinions with regard to an endowment made for the mosque by a Thimmi. The majority of the scholars are of the opinion that it is valid as per the general evidence for endowment, whereas the Maaliki School of jurisprudence prohibited it...”
Therefore, according to the view of the majority of the scholars, it is permissible for a Christian to participate in building a shop for the mosque, whose profit will be allocated according to the specification of the Christian who made the endowment.
Finally, we advise you to strive to call that Christian man to Islam, as Allaah may guide him thanks to you, and you will be a reason of saving him from Hellfire.
Allaah Knows best.