Does anything become incumbent upon me if I had intended not to deal with a specific cashier in a grocery store — or swore by divorce to that effect without being certain — and then dealt with him later unintentionally?

An oath is not contracted by mere intention; rather, it must be uttered verbally. If there is doubt about its utterance, the original state is the acquittal of responsibility. If you have sworn an oath, what determines its intended meaning is what you purposed by it. So, if you intended that the accountant should not calculate the price of the goods, then you do not break your oath by him withdrawing the amount from the card. But if you intended never to deal with him, then you would break your oath by that. It is advised not to give in to obsessive thoughts (waswas).