Sa'id bin al-Musayyib (may Allah have mercy on him) says: Two brothers from the Ansar had an inheritance between them. One of them asked the other to divide the inheritance, and the other said: If you ever ask me for the division again, I will never speak to you, and I will spend all my wealth on the building of the Ka'bah. Sayyiduna Umar (may Allah be pleased with him) said: The Ka'bah has no need of your wealth. Give the expiation for your oath and speak to your brother, for I heard the Prophet (peace and blessings be upon him) say: "There is no oath or vow in what displeases Allah; and in severing ties of kinship and in what one does not own there is no oath either."
Fatwa: The fatwa of Sayyiduna Umar (may Allah be pleased with him) requiring expiation is evidence that what is meant by "there is no oath" in the report is that, when the oath concerns disobedience, there is no oath which one is ordered to keep and fulfil; it does not mean that expiation is not due upon breaking it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19857