If a person said, "If I possess such-and-such piece of land, I will build a mosque on it," but when he possessed it, he became in need of the price of the mosque. Is he obliged to build the mosque on that piece of land or can he cancel his intention?

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 is His slave and Messenger.

If the person said that statement with the intention of making a vow, then it is incumbent on him to deliver on his vow. This is because a vow is considered valid and binding when the person utters an implicit statement with the intention to make a vow, exactly as it is considered valid and binding when explicitly expressed, as agreed upon by the scholars.

Al-Khateeb Ash-Shirbeeni said about the pillars of a vow:

"The formula of the vow should indicate commitment. A vow is not considered valid and binding by mere intention just like other contracts. When a mute person makes understandable gestures indicating that he is making a vow, then his vow is considered valid and binding. Imaam An-Nawawi underlined that a vow is considered valid and binding when the person utters an implicit statement as long as he holds the intention to make a vow. Al-Awza‘i said, "A vow is more likely to be considered valid and binding based on the understandable gesture of a mute person than a contract of sale." [Mughni Al-Muhtaj]   

Nevertheless, if the speaker does not intend to make a vow and merely wanted to utter this statement as an expression of his intention, then it is not incumbent on him to do anything in this regard, and knowledge of that lies with Allaah Almighty.

Allaah Knows best.