What should I do about an oath I swore but cannot recall its intention, or if it even occurred? How can I atone for it by feeding someone I do not know? And what is the meaning of "from the average of what you feed"?

The fundamental principle is the presumption of innocence. So, if you doubt whether an oath was uttered, or whether it was a vain oath (laghw) or a binding one (mun'aqidah), or if you doubt breaking it, then nothing is incumbent upon you. As for the destitute (masakin), they are the needy who do not find enough to suffice them. It is sufficient to have a strong presumption that those to whom it is given are indeed needy. It is permissible to pay the expiation to sanitation workers if their wages are insufficient for them. What is meant by "average" (al-wasat) is the mean between extravagance and stinginess, neither the highest nor the lowest.