The 72nd mitzvah is that in the case of certain transgressions, we are commanded to bring an offering of adjustable value.

The transgressions which require one to bring such a sacrifice are: tumas mikdash, tumas kodshav, taking a false oath (sh'vuas bitui) and swearing falsely that one does not know any testimony (sh'vuas ha'eidus).

In a case of tumas mikdash, when one became tameh through a prime source of tumah (av hatumah) — as we explained in our introduction to Seder Taharos,1 — and entered the Holy Temple unintentionally; or tumas kodshav, when one ate sacrificial meat unintentionally [when he was in a state of tumah]; or when one swore a sh'vuas bitui2 and unintentionally did not fulfill one's promise; or when one swore a false sh'vuas ha'eidus,3 whether unintentionally or on purpose — in any of these cases one must bring an offering of adjustable value.

This commandment derives from G‑d's statement4 (exalted be He), "If a person sins by hearing an oath [to testify, and he does not do so]...or a person who touches anything which is tameh [and then eats something sanctified or enters a sanctified area] not realizing...[and he later discovers that he has committed a violation]; or a person who makes a verbal oath...and then forgets about it...when he is guilty [in any of these cases]...he must bring his guilt-offering...and if he cannot afford [a sheep, he shall bring two birds]...and if he cannot afford [two birds, he shall bring a meal-offering]...."

This is why it is called [an offering of] adjustable value: because it doesn't have a designated species, but the offering is sometimes brought from one species and sometimes from another species — according to the financial ability of the one who transgressed and must bring the offering.

The details of this mitzvah are explained in tractate Kerisus5 and in tractate Shavuos.6