Is it permissible, Islamically, to demolish a mosque under construction on land donated by someone who does not own it, and which its true owner has claimed?

It is not permissible to build a mosque on usurped or stolen land. The owner of the land has the right to demand the demolition of whatever has been built on it without their permission, and this is obligatory, even if the building is a mosque. If the mosque remains despite the landowner's objection, then prayer in it is not permissible by consensus. Scholars have differed on the invalidity of the prayer and the obligation to repeat it. Based on this, the demolition of the aforementioned mosque is not merely permissible, but obligatory.