What is the ruling of the Sharia on someone who intended to build a mosque on land he owns, but does not possess the building costs? And is his intention considered a vow from which he cannot retract, such as by selling the land and donating its price to build other mosques?

A vow (nadhr) is an undertaking of an act of worship that is not originally obligatory in Islamic law. It is not binding unless expressed verbally. Therefore, as long as this person has not uttered the vow, nothing is binding upon him. He may do with the plot of land as he wishes. However, it is preferable for him not to use it except for purposes similar to what he had intended it for.