'Duty' in the Bible

“Again, you have heard that it was said to the men of old, ‘You shall not make false vows, but you shall fulfill your vows to the Lord [as a religious duty].’

And [you scribes and Pharisees say], ‘Whoever swears [an oath] by the altar, that is nothing (non-binding), but whoever swears [an oath] by the offering on it, he is obligated [as a debtor to fulfill his vow and keep his promise].’