In YD Siman 151 it discusses the things that one is allowed and not allowed to sell to a gentile (for reasons that they might be used for Avodah Zara -- idol worship). I didn't see the following particular case discussed in Shulchan Aruch, although perhaps someone will find a reason why it's a problem.
If someone outside of Eretz Yisrael has a large piece of land that a gentile wants to purchase for the purpose of building on it a church or a school to train Christian leaders (or perhaps to build a building for actual idol worship), is one allowed to sell the land? Again it's not a building or house that is being sold, just land, and this is outside of Eretz Yisrael. May he sell the land?