"any sin that people usually commit against one another"
# is unfaithful to me
If someone sins against another person, it means that they have also sinned against Yahweh and Yahweh considers that person as unfaithful to him. AT: "they have also wronged me" (See: [[rc://en/ta/man/translate/figs-explicit]])
# the price of his guilt
Here the person's "sin" is referred to as his "guilt." AT: "what is considered a suitable amount of money for the wrong that he has done" (See: [[rc://en/ta/man/translate/figs-metonymy]])
# add to the price one-fifth more
This means the person must pay an extra one-fifth of the price that he owes.