Gemara

(Mishnah): If Reuven returns a debt to Shimon in Shemitah, Shimon must say 'I am Meshamet (release you from your obligation)'. If Reuven says 'even so (I want to repay)', Shimon may take the money.

We learn this from "this is the Devar (matter, or word) of Shemitah."

(Rabah): Tali Lei (Shimon may coerce Reuven) until Reuven says so.'

Question (Abaye - Beraisa): When Reuven pays, he may not say 'I pay the loan.' Rather, he says that it is a gift.

Answer (Rabah): Yes! Shimon is Tali Lei until Reuven says this.

Aba bar Minyomi returned a debt to Rabah in Shemitah. Rabah said 'I am Meshamet.' Aba took his money and left. Abaye noticed that Rabah was sad; Rabah explained what happened.

Abaye told Aba that he should have said that he wants to pay anyway, and that he should now return the money and say this. Aba did so.

Rabah: He did not know enough to do so the first time.

Mishnah (Shevi'is 10:9): Chachamim are pleased with one who returns a debt after Shemitah.

Rishonim

Rambam (Hilchos Shemitah 9:28): Chachamim are pleased with one who returns a debt after Shemitah.

Mishnah Rishonah (Shemitah 10:9): Since it is a mere gift, why are Chachamim pleased with him? Perhaps it is so people will not refrain from lending. Alternatively, since the Torah allows him to say 'even so...', this shows that the Mitzvah is primarily that the lender not press for payment. It is a Mitzvah to repay the debt, like regular debts.

Note: It is forbidden to say that he repays it like a debt!

Rambam (ibid.): If Reuven returns a debt to Shimon after Shemitah, Shimon must say 'I am Meshamet. You are exempt.' If Reuven says 'even so, I want you to accept from me', Shimon may take the money. It says "do not press (for payment)", and Shimon did not press him. Reuven may not say 'I am paying the loan.' Rather, he says 'it is my money; it is a gift.'

Rambam (29): If Reuven returned a debt but did not say so, Shimon verbally pressures him until he says 'it is mine; I gave it to you for a gift.' If Reuven does not say so, Shimon may not accept. He takes his money and goes away.

Ri Korkus: This is close to the Aruch, who explains 'Tali Lei' to mean that he stares at him like one who wants to receive, until Reuven is embarrassed and says so. (This is the Rishonim's text of the Aruch. Our text is like Rashi.)

Me'iri (DH me'Atah): Shimon may verbally detain Reuven as if he hangs him, so he will not leave with his money. He verbally pressures him, saying how he lent to him out of love, but the Torah commands him to release, but Reuven may repay if he wants, just he informs him that he cannot force him... until Reuven agrees to return it for a gift. Some say that he stares at the wallet until Reuven understands that he wants the money and is embarrassed. If Reuven goes away, Shimon may express his pain to others, and they themselves may tell Reuven that he acted improperly. Shimon may not order them to do so.

Rosh (4:19): If Reuven returns a debt to Shimon in Shemitah, i.e. after Shemitah, in a period when Shemitah applies, Shimon must say 'I am Meshamet.' We learn from "this is the Devar of Shemitah." Once one says a word (that he refrains), he was Yotzei Shemitah. Rashi explains 'Tali Lei ' to mean that if Shimon is able, he hangs Reuven on a tree. This is unreasonable. If so, Shemitah is Batel (the lender can force the borrower to repay)! Rather, Shimon stares at Reuven until Reuven says that he wants to repay. The Yerushalmi supports this. It says that Shimon says 'I refrain' solemnly, with his right hand stuck out to receive.

Bach (CM 67 DH v'Lo): This is not difficult for Rashi. Shimon may hang Reuven only after he offered to repay and Shimon fulfilled Shemitah by saying 'I am Meshamet.' Also the Terumas ha'Deshen (304) says so. One who is lenient like Rashi is zealous and profits.

Note: The Terumas ha'Deshen does not permit to be lenient like Rashi.

Drishah (35): According to the Rosh, what was Abaye's question? Surely, the Beraisa means that the lender stares at him until he says that he wants to repay! This forced Rashi to explain that he hangs him until he agrees. Abaye held that this cannot be considered a gift! The Rosh must hold that Abaye held that even when he pressures him through staring, it is not called a gift!

Hasagos ha'Ra'avad (under Rif, 19b): Some say that Shimon grabs Reuven by his garment until he says so. This is unreasonable. Rather, after Reuven says 'even so', Shimon does not stash away the money. Rather, Shimon suspends the money in front of him, lest it seem that he did not firmly resolve to cancel the loan. Abaye challenged Rabah, for a Beraisa obligates Reuven to say that he gives a gift, so surely Shimon need not suspend the money! Rabah answered that he must suspend it when Reuven said 'even so', but did not yet say that it is a gift. This was when Shemitah applied. In the days of Rabah and the latter Amora'im, it was mere Midas Chasidus. Rabah took back a loan after Aba bar Minyomi said only 'even so', and considered Aba to be ignorant for not knowing that he should return them.

Note: The Shulchan Aruch (CM 67:1) rules that Shemitah applies mid'Rabanan nowadays, like most Poskim. The Rema says that some say that it does not apply at all nowadays.

Mordechai (380): If Reuven did not come to return the money, there is no need for Shimon to say 'I refrain.'

Poskim

Shulchan Aruch (CM 67:36): If Reuven returns a debt to Shimon after Shemitah, Shimon must say 'I am Meshamet. You are exempt.' If Reuven says 'even so, I want you to accept from me', Shimon may accept. Reuven may not say 'I pay the loan.' Rather, he says 'it is my money; it is a gift.' If Reuven returns a debt but does not say so, Shimon verbally pressures him until he says so. If Reuven does not say so, Shimon may not accept. He takes his money and goes away.

SMA (58): Reuven takes his money and goes away.