Even though the money that a Tzaddik receives from an evil individual might be stolen money, the Tzaddik is permitted to accept it. This is so that this individual does not find himself some wicked scholar to call a Tzaddik, give him the money, and then oppose the true Tzaddik.
א. מַה שֶּׁהַצַּדִּיק מְקַבֵּל מָמוֹן מֵהָרָשָׁע, אַף־עַל־פִּי שֶׁזֶּה הַמָּמוֹן הוּא סָפֵק גֶּזֶל, אַף־עַל־פִּי־כֵן מֻתָּר לְקַבֵּל מִמֶּנּוּ, כְּדֵי שֶּׁלּא יְקַבֵּל הָרָשָׁע לְעַצְמוֹ אֵיזֶהוּ תַּלְמִיד חָכָם רָשָׁע, וְיּאמַר עָלָיו שֶׁהוּא צַדִּיק, וְיִתֵּן לוֹ הַמָּמוֹן, וְיַחֲלק עַל הַצַּדִּיק הָאֱמֶת.
Even though the money that a Tzaddik receives from an evil individual might be stolen money, the Tzaddik is permitted to accept it. This is so that this individual does not find himself some wicked scholar to call a Tzaddik, give him the money, and then oppose the true Tzaddik.