Does not sending money to his friend—despite his intention to do so—constitute a breach of the oath he swore to his colleague regarding the unavailability of money, and is expiation for the oath incumbent upon him?

Regarding the oath that you need money to complete your house: If you are indeed in need, then there is nothing upon you. But if you are not in need, then it is a yamin ghamus (a false oath sworn knowingly), and what is obligatory in this case is repentance, and there is no expiation for it according to the majority of scholars.

However, if the oath was to send money and you were unable to send it: This is considered laghw al-yamin (a vain or unintentional oath) related to a future event, and it is more prudent that you offer an expiation for an oath.