Negative commandment · Lo Ta’aseh #249 of 613
Not to swear falsely in denying a monetary [obligation to] a colleague
שלא לישבע על כפירת ממון חברו
The verse · Vayikra 19:11
לֹא תִּגְנֹבוּ וְלֹא־תְכַחֲשׁוּ וְלֹא־תְשַׁקְּרוּ אִישׁ בַּעֲמִיתוֹ
“You shall not steal, neither deal falsely, neither lie one to another.”
The Rambam’s words
Not to swear falsely in denying a monetary [obligation to] a colleague, as [Leviticus, ibid.] states: "Do not swear [falsely]" - i.e., do not swear falsely about money owed to a colleague.
שלא לישבע על כפירת ממון חברו, שנאמר "לא תשקרו" (ויקרא יט,יא); כלומר, לא תישבע על שקר בממון שיש לחברך בידך.
Why · Sefer HaChinuch #226
To not swear about the denial of money
“It is from the roots of this commandment [that it is] for people to know and fix in their souls and strengthen the faith in their hearts about G-d, blessed be He — Who is in the Heavens above and exists forever — that there is nothing else like His existence.”
Who & when
“And it is practiced in every place and at all times.”
Where in the Mishneh Torah
Hilchot Oaths הלכות שבועות