Is my oath still valid regarding not topping up my mobile balance until I return to my country, knowing that I intended it to curb extravagance, and that the return date I had in mind at the time of the oath has changed?

What is taken into account in oaths is the intention of the swearer at the time of swearing. So, whenever he intends with his oath something that the wording can bear, his oath is tied to what he intended, not to the wording. If he did not intend anything, then reference is made to the reason for the oath and what provoked it. So, if a person swears to do or not to do something at a specific time according to his intention and belief, and then the time changes against his will, what is considered in determining the time is what was in his intention, and he does not break his oath if the intended time is delayed.