7. In accordance with rules of canon law, an oath extorted by violence or grave fear is valid, but it can be dispensed by an ecclesiastical superior. Moreover, an oath taken without violence or fraud by which one renounces a private advantage or favour granted him by law must be kept as long as it can be kept without sin (Can. 1317, sec. 2, 3).
8. An oath may cease to bind from intrinsic or from extrinsic causes. If circumstances produce a change in the matter of the oath so that it has become unlawful, or useless, or an obstacle to greater good, or if some condition is not fulfilled, the oath no longer binds. The same must be said when the motive of a promissory oath no longer exists, as if I swore to help a poor person with money who subsequently becomes rich (Can. 1319).
An oath may be annulled, dispensed, commuted, or relaxed, in much the same way as a vow, and it will be more convenient to treat of these extrinsic causes for being released from the obligation of an oath in the next chapter, where the doctrine is applicable to vows and oaths alike (Can. 1320).
