Mishnayos Shevuos Perek 8 Mishnah 6

But if the owner said to the borrower: Where is my ox? And the borrower said to him: I do not know what you are talking about, but the truth was that it died or was injured or captured or stolen or lost, and the owner said: I administer an oath to you concerning your claim, and the borrower said: Amen, the borrower is liable to bring a guilt-offering, as he took an oath that would render him exempt from liability to pay. If an owner said to a paid bailee or a renter: Where is my ox? And the latter said to him: It died, but the truth was that it was injured or captured; or if he said: It was injured, but the truth was that it died or was captured; or if he said: It was captured, but the truth was that it died or was injured; or if he said: It was stolen, but the truth was that it was lost; or if he said: It was lost, but the truth was that it was stolen, and the owner said: I administer an oath to you concerning your claim, and he said: Amen, the paid bailee or renter is exempt from bringing a guilt-offering. If the paid bailee or renter said: It died or was injured or captured, but the truth was that it was stolen or lost, and the owner said: I administer an oath to you concerning your claim, and he said: Amen, the paid bailee or renter is liable to bring a guilt-offering. If the paid bailee or renter said: It was lost or stolen, but the truth was that it died or was injured or captured, and the owner said: I administer an oath to you concerning your claim, and he said: Amen, he is exempt from bringing a guilt-offering. This is the principle: Anyone who changes from one claim of liability to another claim of liability or from one claim of exemption to another claim of exemption or from a claim of exemption to a claim of liability is exempt from bringing a guilt-offering. If he changes from a claim of liability to a claim of exemption, he is liable. This is the principle: Anyone who takes an oath to be lenient with himself is liable; if he takes an oath to be stringent with himself, he is exempt.

הֵיכָן שׁוֹרִי, אָמַר לוֹ אֵינִי יוֹדֵעַ מָה אַתָּה סָח, וְהוּא שֶׁמֵּת אוֹ נִשְׁבַּר אוֹ נִשְׁבָּה אוֹ נִגְנַב אוֹ אָבַד. מַשְׁבִּיעֲךָ אָנִי, וְאָמַר אָמֵן, חַיָּב. אָמַר לַנּוֹשֵׂא שָׂכָר וְהַשּׂוֹכֵר הֵיכָן שׁוֹרִי, אָמַר לוֹ מֵת, וְהוּא שֶׁנִּשְׁבַּר אוֹ נִשְׁבָּה. נִשְׁבָּר, וְהוּא שֶׁמֵּת אוֹ נִשְׁבָּה. נִשְׁבָּה, וְהוּא שֶׁמֵּת אוֹ נִשְׁבַּר. נִגְנָב, וְהוּא שֶׁאָבַד. אָבַד, וְהוּא שֶׁנִּגְנַב. מַשְׁבִּיעֲךָ אָנִי, וְאָמַר אָמֵן, פָּטוּר. מֵת אוֹ נִשְׁבַּר אוֹ נִשְׁבָּה, וְהוּא שֶׁנִּגְנַב אוֹ אָבַד. מַשְׁבִּיעֲךָ אָנִי, וְאָמַר אָמֵן, חַיָּב. אָבַד, אוֹ נִגְנָב, וְהוּא שֶׁמֵּת אוֹ נִשְׁבַּר אוֹ נִשְׁבָּה. מַשְׁבִּיעֲךָ אָנִי, וְאָמַר אָמֵן, פָּטוּר. זֶה הַכְּלָל, כָּל הַמְשַׁנֶּה מֵחוֹבָה לְחוֹבָה וּמִפְּטוֹר לִפְטוֹר וּמִפְּטוֹר לְחוֹבָה, פָּטוּר. מֵחוֹבָה לִפְטוֹר, חַיָּב. זֶה הַכְּלָל, כָּל הַנִּשְׁבָּע לְהָקֵל עַל עַצְמוֹ, חַיָּב. לְהַחְמִיר עַל עַצְמוֹ, פָּטוּר: