We are in chapter 5 of Maseches Shevuos, mishnah 2. The mishnah sets out to clarify the nature of a shevuas hapikadon: an oath in which the claimant asserts that an item of his is in his fellow's possession, and the defendant denies it and swears that the deposit is not in his hands.
"Shevuas hapikadon keitzad" - how does the oath concerning a deposit work?
"Amar lo: ten li pikdoni sheyesh li b'yadecha" - the claimant demands the item he deposited with his fellow for safekeeping. "Shevuah she'ein lecha b'yadi" - the defendant denies it and swears on his own initiative that he holds nothing of his.
"Ein lecha b'yadi" - the defendant denies the claim without mentioning an oath, and the claimant responds: "Mashbia ani alecha" - meaning, I want you to swear to your claim. "V'amar: amen" - the defendant accepts the oath upon himself by answering amen.
"Harei zeh chayav" - in both cases the one who swears becomes liable to bring an asham offering, as explained in the previous mishnah.
Multiple oaths and multiple offerings:
The mishnah continues: "Hishbia alav chamishah pe'amim, bein bifnei beis din u'vein shelo bifnei beis din, v'chafar - chayav al kol achas v'achas" - if the claimant administered the oath to him five times, whether in court or outside of court, and each time the defendant denied the claim and swore to his denial, he is liable to a separate offering for each and every denial.
Rabbi Shimon's reasoning:
Rabbi Shimon explains: "Mah taam? Mipnei sheyachol lachzor u'lehodos" - what is the reason? Because he is able to retract and admit. After each denial, the defendant had the option of retracting and admitting that he owes the money. Since he did not do so, and denied it again, this is a fresh and independent denial, for he had the opportunity to admit. It turns out that he is denying a monetary claim with each and every oath, and therefore he is liable to an offering for each one on its own.
The difference from shevuas ha'edus:
In the previous chapter, regarding the oath of testimony, Rabbi Shimon ruled that the one who swears cannot retract and admit. The reason is that in the oath of testimony, which takes place before a court, the witnesses' denial is equivalent to the giving of testimony itself, and once they have testified they can no longer retract that testimony. Here, however, we are not dealing with testimony but with the denial of a monetary claim, and therefore each denial stands on its own, the oath takes effect on each and every one, and hence there is liability to an offering for each and every one.
In summary: in this mishnah we learned the definition of shevuas hapikadon in its two forms - swearing on one's own initiative, or answering amen after someone else administers the oath - and in both the person is liable to an asham offering. We also learned that one who administers the oath to his fellow several times, in court or outside of court, is liable for each denial separately, since with each and every oath he could have retracted and admitted, and so he is found to be denying the money anew. In this, shevuas hapikadon differs from shevuas ha'edus, where the denial is equivalent to the giving of testimony and no retraction is possible thereafter.