Does one have to fast if he swore that something belonged to a certain person, and then discovered that it did not belong to him?

If you swore an oath believing the water belonged to one colleague, and then it turned out to belong to another, then according to most scholars, you are not required to offer expiation (kaffarah). This is because it falls under the category of a "laghw al-yameen" (unintentional or vain oath), and Muslims have reached a consensus on this, based on the Almighty's saying: "Allah will not impose blame upon you for what is unintentional in your oaths." However, if you wish to offer expiation as a precaution, that is good and more absolving for your conscience. Know that the expiation for an oath (kaffarah al-yameen) cannot be fulfilled by fasting unless one is unable to feed ten needy persons, or clothe them, or free a believing slave.