Shevuot (Oaths) 49 - Four Custodians Who Swear

Any of the four types of custodians may swear regarding the object he was keeping. Even if he swears falsely, he is not necessarily liable for a false oath.

If his oath would change his obligation from one liability to another, such as that the ox he borrowed died naturally, when actually it was captured, and in both of which cases he would be liable to pay, his oaths changes nothing. If he confesses that he swore falsely, he does not have to bring the usual sacrifice nor pay one-fifth extra. The same would apply if he swears to change from one exemption to another, or from an exemption to a liability.

However, if he swears to change from liability to exemption, but then confesses, he needs to bring a guilt-offering and must add one-fifth to the amount.

Art: Joaquin Sorolla y Bastida - Oxen on the beach