A derivative law should never be stricter than its source/ it is sufficient for the law derived by inference to be as strict as the law whence it is derived
A matter derived from an analogy to another matter cannot be stricter than the source from which it was learned and to which it was compared. When one learns one case from another by "kal va-chomer" (a fortiori reasoning), it is impossible that the law of the derived matter ('the one coming from the law') is stricter than the explicit source ('the subject under judgment').
(For example, to say that hand agency requires no deficiency, and likewise: one should not say 'it is sufficient for the law derived by inference to be as severe as the original subject' — Bava Metzia 45b.)
Translation
Expression (Hebrew)
דַּיּוֹ לַבָּא מִן הַדִּין לִהְיוֹת כְּנִדּוֹן
Definition (Hebrew)
דָּבָר שֶׁהֻסַּק מֵהַשְׁוָאָה לְדָבָר אַחֵר אֵינוֹ יָכוֹל לִהְיוֹת חָמוּר יוֹתֵר מֵהַמָּקוֹר שֶׁמִּמֶּנּוּ הוּא נִלְמָד וְשֶׁאֵלָיו הוּא הֻשְׁוָה. כְּשֶׁלּוֹמְדִים עִנְיָן אֶחָד מְעַנְיֵן אַחֵר ב"קַל וָחֹמֶר", לֹא יִתָּכֵן שֶׁיִּהְיֶה דִּינוֹ שֶׁל הַדָּבָר שֶׁלָּמַדְנוּ ('הַבָּא מִן הַדִּין') חָמוּר יוֹתֵר מִן הַמָּקוֹר הַמְּפֹרָשׁ ('הַנִּדּוֹן').
(לוֹמַר לָךְ שְׁלִיחוּת יָד אֵין צְרִיכָה חֶסְרוֹן, וְאִידָךְ: שֶׁלֹּא תֹּאמַר דַּיּוֹ לַבָּא מִן הַדִּין לִהְיוֹת כַּנִּדּוֹן"- בָּבָא מְצִיעָא מָא ע"ב)