What is the ruling of Islamic law concerning swearing to pay the bill, then another person insists on paying it?

If, by your oath, you intended that none of your friends present at the time of the oath should pay the bill, then you do not break your oath if someone who arrived later pays, because the reference point for oaths is the intention. However, if you intended that no one other than you should pay the bill whatsoever, then you break your oath if you do not pay it, and expiation becomes incumbent upon you at that time.