Is swearing not to pay 3.30 EGP to a telecom company—based on the belief that it is not due—then paying it after learning it is due, considered an oath (yamin ghamus)?

This oath is not a Ghamoos oath. The Ghamoos oath is swearing falsely and intentionally about a past event, and it immerses the swearer in sin, with no expiation for it. If you swore not to pay an amount because you thought it was not due from you, you do not break your oath by paying it if it becomes clear that it is due. This is because oaths are based on the intention of the swearer and their cause. So, if the swearer had no specific intention, but there was a cause that provoked the oath, then their ruling is like that of someone who intended it, because the cause is an indication of the intention and purpose.