To make a bad bargain ("to exchange a cow for a donkey")
In the question: Make a bad deal (a cow is considered more valuable than a donkey).
("The one who exchanges a cow for a donkey and it gives birth, and the one who sells its offspring and it gives birth, this is what I say, until I sell it, and this is what I say, from the time I take it, they shall divide it" - Baba Matzi'a 8:4)
Translation
Expression (Hebrew)
הֶחְלִיף פָּרָה בַּחֲמוֹר
Definition (Hebrew)
בְּהַשְׁאָלָה: עֲשֵׂה עִסְקָה גְּרוּעָה (פָּרָה נֶחְשֶׁבֶת לִיקָרָה יוֹתֵר מֵחֲמוֹר).
("הַמַּחֲלִיף פָּרָה בַּחֲמוֹר וְיָלְדָה, וְכֵן הַמּוֹכֵר שִׁפְחָתוֹ וְיָלְדָה, זֶה אוֹמֵר עַד שֶׁלֹּא מָכַרְתִּי וְזֶה אוֹמֵר מִשֶּׁלָּקַחְתִּי, יַחֲלֹקוּ"- בָּבָא מְצִיעָא ח ד)