Every seventh year is a Shmitah year, when the land is allowed to lie fallow and any fruit that does grow is considered ownerless and free for the taking(Shmitat Karkaot), and when loans are forgiven(Shmitat Ksafim). In his paper "מצוות שמיטת כספים" in מנחת אביב, Rav Aharon describes the well known chakira on Shmitat Ksafim.
Machloket: How Chov is Eliminated
With the very concise wording of Torah commandment commandment to forgive loans, various questions arise: When is the loan actually cancelled? Does the loan still exist if one doesn't forgive it? Rav Aharon cites three opinions among the Rishonim on the topic.
1. Lender Forgives Loan
The Yereim says that the loan only ceases to exist once the lender explicitly forgives it.
וחוב שעבר עליו שביעית אינו רשאי לווה לעכבו אלא על פי מלוה, שכל זמן שלא השמיטו מלוה חייב לפרוע...(יראים סי’ קסד)
The Ohr Zaruah, Rambam, and Ramban, on the other hand, say that the loan is cancelled at the end of the Shmitta year, regardless of the will of the lender.
(אור זרוע ע"ז סי’ קח, רמב"ם שמיטה ט:ד, רמב"ן בספר התרומות מה:ה)
The Ittur's opinion essentially combines these previous two ideas. During Shmitta year, a lender is commanded to forgive the loan and only once he does is it cancelled. At end of year, the Torah forgives any remaining loans regardless.
והא דתנן המחזיר חוב לחבירו צריך לומר לו משמט אני, דווקא בשביעית, אבל לאחר שביעית לא(העיטור אות פ פרוזבול)
Limit on Afkata Demalka
Rav Aharon ultimately argues(based on the Mishna in Shviit 10:8 and Gamara in Gittin 37B) that, even according to the opinion that the loan is automatically nullified, it is really more frozen than cancelled and still remains intact on some level. Only once the lender fulfills the commandment and verbally forgives the loan does it disappear entirely.