An oath sworn to deceive someone may cause forfeiture of Faith ('Iman)
Verse 94 which begins with the words: وَلَا تَتَّخِذُوا أَيْمَانَكُمْ دَخَلًا (do not take your oaths as a means of mischief) gives yet another instruction to stay away from getting involved in what is a great sin laden with evil consequences. It means that anyone who at the very time of taking an oath harbours other ideas and intentions, is all set to act counter to the oath taken while going through the exercise of oath-taking merely to deceive the other party, then, this would be an act far more dangerous than that of breaking a common oath. As a result, the danger is that this person may end up being all deprived of the very asset of'Iman (faith). The sentence which follows immediately, that is: فَتَزِلَّ قَدَمٌ بَعْدَ ثُبُوتِهَا (lest a foot should slip after it is firmly placed - 94), means exactly this. [ Qurtubi ]