וַיֹּ֗אמֶר הִנֵּֽה־נָ֤א הוֹאַ֙לְתִּי֙ לְדַבֵּ֣ר אֶל־אֲדֹנָ֔י אוּלַ֛י יִמָּצְא֥וּן שָׁ֖ם עֶשְׂרִ֑ים וַיֹּ֙אמֶר֙ לֹ֣א אַשְׁחִ֔ית בַּעֲב֖וּר הָֽעֶשְׂרִֽים׃
И сказал он: 'Вот, теперь я взял на себя, чтобы говорить с Господом. Там должно быть двадцать найденных приключений.' И сказал он: 'Я не уничтожу его за двадцать'ради'
Rashi
הואלתי means I am willing to speak, just as (Exodus 2:21) “And Moses was pleased (ויואל) [to dwell with the man]”.
Sforno
לא אשחית בעבור העשרים...בעבור העשרה. By destroying 3 out of the five cities the other two will not escape the retribution completely, seeing that the satellite towns are by definition largely dependent on the major urban center. Its destruction will cast its shadow on the satellite towns, as we know from Berachot 58, i.e. נתקללה בבל נתקללה בנותיה, when Babylon is cursed so are its satellite towns. (according to Rashi there: אוי לרשע אוי לשכניו, when the wicked experiences woes, so do his neighbours.”)