Arachin, Chapter 5, Mishnah 6. Throughout this chapter the Mishnah has been occupied with the ways the Temple treasury collects what is due to it, and here we meet a very practical difficulty: the debtor who takes his time.
A person carrying an obligation toward the Sanctuary, whether an erech he declared upon himself or an offering he became liable to bring, must move quickly to discharge it. He should not allow another regel to arrive while the debt still stands. Should an entire year slip by, with Pesach, Shavuot and Sukkot all passing while his pledge remains unpaid, he has violated a lo ta'aseh.
How, then, is such a person encouraged to settle his account in time?
Taking Security
The mishnah rules: "Chayavei arachin memashkenin otan" - one who is liable for an erech has a pledge taken from him. The gizbar, the officer in charge of the treasury, takes hold of property belonging to the man who made the vow and keeps it in the treasury's hands; when payment arrives, the property returns to its owner.
"Chayavei chatatot va'ashamot ein memashkenin otan" - no security is taken from someone who still owes a chatat or an asham. What accounts for the distinction? Offerings of that sort follow upon an aveirah, and the man who owes one understands perfectly well that his kaparah waits on it. He is hardly going to drag his feet; he will come to the Beit Hamikdash at the earliest opportunity. Pressure is unnecessary.
"Chayavei olot ushlamim memashkenin otan" - security is taken from one who has an olah or a shelamim outstanding. Both of these are freely undertaken; nobody imposed the obligation on him. And precisely because it grew out of his own initiative, as with an erech, there is reason to worry that it be paid on schedule, so the treasury holds property of his as a guarantee.
"I Am Willing"
Now, an offering of this kind accomplishes its purpose for its owner only if he brings it of his own accord: "af al pi she'eino mitkaper lo", no atonement is effected for him, "ad sheyitratzeh", until he brings it with genuine consent, "shene'emar: lirtzono", since the Torah itself stipulates that the offering come by his own desire. Even so, the treasury may hold a security deposit in order to press him over the shelamim or olah he took upon himself.
"Kofin oto" - he is compelled, "ad sheyomar: rotzeh ani", until the words "I want to" come out of his mouth. The moment he says them, the offering is regarded as having been brought by his own will, despite the coercion that preceded the statement.
The Parallel to Gittin
"Vechen atah omer begitei nashim" - the identical rule, teaches the mishnah, operates in the laws of divorce. A get takes effect only if the husband hands it over willingly. Nonetheless, there are circumstances in which he is pressed to do so, and the get that results is not thereby ruined. Here as well the same formula holds: he is squeezed "ad sheyomar: rotzeh ani", until he states that he wishes to give it. From that point his act counts as voluntary, and the get is halachically valid.
The Rambam's Insight
The Rambam draws out a lovely idea here. Deep down, every Jew desires to do the right thing. The man who promised money to the Sanctuary really does wish to hand it over. The husband who ought to release his wife recognizes that giving her the get is proper. The obstacle is the yetzer hara, which blocks him from carrying out what he himself truly wants. A nudge from the outside, then, does nothing more than move him in the direction his own heart is already pointing.
That is the logic behind compelling a husband in the matter of a get, which naturally is done only under the authority of a beit din. When he finally agrees, the agreement is authentic, because it gives voice to the genuine inner desire of a Jew to act rightly.