Those who say that the expiation is to not fulfill it
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ عَبْدِ الْمُؤْمِنِ الْوَاسِطِيُّ، حَدَّثَنَا عَوْنُ بْنُ عُمَارَةَ، حَدَّثَنَا رَوْحُ بْنُ الْقَاسِمِ، عَنْ عُبَيْدِ اللَّهِ بْنِ عُمَرَ، عَنْ عَمْرِو بْنِ شُعَيْبٍ، عَنْ أَبِيهِ، عَنْ جَدِّهِ، . أَنَّ النَّبِيَّ ـ صلى الله عليه وسلم ـ قَالَ " مَنْ حَلَفَ عَلَى يَمِينٍ فَرَأَى غَيْرَهَا خَيْرًا مِنْهَا فَلْيَتْرُكْهَا فَإِنَّ تَرْكَهَا كَفَّارَتُهَا " .
It was narrated from 'Amr bin Shu'aib, from his father, that his grandfather said that the Prophet (ﷺ) said: "Whoever swears an oath then sees that something else is better than it, let him not do it, and his leaving it is the expiation for it. "