The Prophet ﷺ said: If anyone says when swearing an oath: "If Allah wills," he makes an exception. (Using translation from Abū Dāʾūd 3261)   

أَنَّ النَّبِيَّ ﷺ قَالَ مَنْ حَلَفَ عَلَى يَمِينٍ ثُمَّ قَالَ إِنْ شَاءَ اللَّهُ فَقَدِ اسْتَثْنَى  

[Machine] The Messenger of Allah ﷺ said, "Whoever takes an oath by a right hand and then says, 'If Allah wills,' then he is choosing the best way. And if He wills otherwise, then he will not do it."  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ مَنْ حَلَفَ عَلَى يَمِينٍ ثُمَّ قَالَ إِنْ شَاءَ اللَّهُ فَهُوَ بِالْخِيَارِ إِنْ شَاءَ فَعَلَ وَإِنْ شَاءَ لَمْ يَفْعَلْ