What should I do about lying to my manager regarding my colleague's whereabouts, along with my denial of swearing by God to that, while my colleagues affirm that I did swear, knowing that I have left the job?

As long as you deny having sworn an oath, the default is that you have not sworn one. One who doubts the origin of an oath is not liable for anything, because the default is absolution, and certainty is not dispelled by doubt. Whoever doubts whether he did something or not, the default is that he did not do it. This includes one who doubts whether he swore by Allah, or by divorce, or by emancipation; such an oath is invalid, so he is not liable for anything. What your colleagues say holds no weight, as long as you are certain that you did not swear an oath. You must offer sincere repentance and seek forgiveness for the lie you committed, for "repentance erases what came before it."