What is the expiation for an oath for someone who swore to take down a billboard, while his colleague swore by his mother's mercy not to take it down, and it was not taken down?

If you swear by Allah Almighty or by one of His attributes, and then break your oath, you are obligated to offer expiation for the oath. This expiation is feeding ten needy persons, or clothing them, or freeing a slave. If you cannot find any of these, then it is fasting for three days, as stated in verse 89 of Surah Al-Ma'idah.