Ustadh Tabraze Azam is asked if it is sinful to delay an expiation fast for breaking an oath.
Is it sinful to delay fast for an oath expiation?
Yes, it is considered to be religiously sinful to delay the mandatory expiation (kaffara) of a nullified oath (yamin), unless you have a reasonable excuse to do so, such as being too poor to make the payment and too sick to fast.
The general basis with duties is that they are to be taken care of as soon as reasonably possible and without undue delay, except if you have a reasonable excuse to do the contrary. At the very least, you should include the expiation (kaffara), in this case, in a document containing any other unfulfilled duties which you owe to Allah Most High.
Allah Most High said, “And hasten towards forgiveness from your Lord and a Paradise as vast as the heavens and the earth, prepared for those mindful of Allah.” (Sura Aal ‘Imran 3:133) The divine injunction here is to rush to that by which you will attain unto forgiveness, such as by fulfilling your duties (wajibat).
Note that if there is an undue delay, you should repent for your error.
(Ibn ‘Abidin, Radd al-Muhtar ‘ala al-Durr al-Mukhtar)
And Allah Most High knows best.