All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The mere promise of building a mosque or declaring the intention to build one does not bind you to the promise, and nothing entails on you, neither expiation nor anything else.
What makes it necessary for a person to act upon and which he cannot go back is if he makes a vows that necessitates commitment, such as saying, 'I vow or I make an oath to Allah to build a mosque', and so on.
Allah knows best.