It is narrated from Sayyiduna Zayd bin Arqam (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "When a man makes a promise to his brother and his intention is to fulfil it, and then he is unable to come at the appointed time, there is no sin upon him."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5061