What is the expiation for an oath sworn not to marry a woman one is already engaged to?

Whoever says: "It is unlawful for me to marry so-and-so," and then marries her, must offer expiation for an oath. This is because rendering lawful things unlawful has the ruling of an oath, as Allah Almighty says: (O Prophet, why do you prohibit what Allah has made lawful for you, seeking to please your wives? And Allah is Forgiving and Merciful. Allah has already ordained for you [believers] the dissolution of your oaths. And Allah is your Protector, and He is the Knowing, the Wise.) At-Tahrim/1, 2.

The expiation for an oath is: freeing a slave, or feeding ten needy persons, or clothing them. And whoever cannot find [the means] should fast for three days.