Be so good as/have the kindness to/please, have the goodness to
He agreed because of the goodness of his heart. He agreed because of his goodness (the first source is in the Tanakh as in Judges 19:6: "And the girl's father said to the man, 'Come here and lie down, and your heart will be glad.'").
("I have despised the acceptance of his precious letter to me, written on the second day of the month of the Lord, and because he was pleased in his goodness to inquire about my well-being and the health of my body" - Shu't Peter Aaron Siman IV, and so: "Many of the surrounding rabbis opened and asked: he was pleased in his goodness and will tell, that the words of the Ustaz will always be desired" - Y. Burla/ Kisuphim, 104, and so: "Grace "Grace that has been poured out in goodness to fill the inhabitants of the garden" - Agnon/ Tamol Shilshom, 113)
Translation
Expression (Hebrew)
הוֹאִיל בְּטוּבוֹ, הוֹאֵל נָא בְּטוּבְךָ
Definition (Hebrew)
הִסְכִּים בִּגְלַל טוֹב לִבּוֹ. נֶעְתַּר בִּגְלַל אָפְיוֹ הַטּוֹב (הַמָּקוֹר הָרִאשׁוֹנִי בַּתָּנָ"ךְ כְּמוֹ בְּשׁוֹפְטִים יט ו:"וַיֹּאמֶר אֲבִי הַנַּעֲרָה אֶל הָאִישׁ הוֹאֶל נָא וְלִין וְיִטַב לִבֶּךָ").
("הִנְנִי בָּזֶה לְקַיֵּם קַבָּלַת מִכְתָּבוֹ הַיָּקָר אֵלַי כָּתוּב מִיּוֹם ב' הע"ל, וְכִי הוֹאִיל בְּטוּבוֹ לִדְרֹשׁ בִּשְׁלוֹמִי וְעַל מַצַּב בְּרִיאוּת גּוּפִי"- שׁוּ"ת פְּאֵר אַהֲרֹן סִימָן ד, וְכֵן:"פָּתְחוּ רַבִּים מִן הַמְּסֻבִּים וּבִקְּשׁוּ: יוֹאִיל בְּטוּבוֹ וִיסַפֵּר,שֶׁדִּבְרֵי הָאוּסְתָּאז תָּמִיד רְצוּיִים"- י.בּוּרְלָא/ כִּסּוּפִים, 104, וְכֵן:"חֵן חֵן שֶׁהוֹאִילָה בְּטוֹבָה לְמַלֹּאות אֶת מְקוֹמִי בַּגַּן"-עַגְנוֹן/ תְּמוֹל שִׁלְשׁוֹם, 113)