Sayyiduna Zayd bin Arqam (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) that the Prophet (peace and blessings be upon him) said: "When someone makes a promise to his brother, intending to fulfil it, but he is unable to fulfil it or cannot come at the appointed time, there is no sin upon him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20838