Arachin 21 - Forcing to Fulfill the Temple Obligation
When one makes a vow to the Temple, whether to donate a symbolic value of a person, or a real monetary value, or any money or property donation, he needs to fulfill that vow in the timely fashion. If he procrastinates, the Temple treasurers may get the permission of the court to exact some of his possessions as security. If he does not pay within thirty days, they become the property of the Temple.
This is also true if he promises a voluntary offering. Since it brings optional atonement, for example, for wrong thoughts or for skipping saying "Shema Israel," one does not feel pressured to fulfill his vow and needs an additional incentive. However, for sin and guilt offerings - which are brought, for example, for unwitting Shabbat violations, or for liaison with a maidservant designated for another - treasurers do not exact security: one who feels the need to atone for this will eventually do it on his own.
Since the Torah said, " He must bring it, of his will ," fulfilling his vow to bring a sacrifice is a commandment, but it must be done willingly. Therefore, if need be, they force him, until he declares "I want to do it."
Art: Philip Hermogenes Calderon - Broken Vows