Resh Lakish said: An ox that killed the slave unintentionally is exempt from the thirty shekels, as it is said, "he shall give thirty shekels of silver to his master, and the ox shall be stoned" (Exodus 21:32): at the time when the ox is subject to stoning, the owners pay thirty shekels; when the ox is not subject to stoning, the owners do not pay thirty shekels.
When the Ox Is Not Stoned the Owner Owes No Thirty Shekels
Yalkut Shimoni on Torah 341:5
אֲמַר רֵישׁ לָקִישׁ, שׁוֹר שֶׁהֵמִית אֶת הָעֶבֶד שֶׁלֹּא בְּכַוָּנָה, פָּטוּר מִשְּׁלֹשִׁים שְׁקָלִים, שֶׁנֶּאֱמַר כֶּסֶף שְׁלֹשִׁים שְׁקָלִים יִתֵּן לַאדֹנָיו וְהַשּׁוֹר יִסָּקֵל, בִּזְמַן שֶׁהַשּׁוֹר בִּסְקִילָה בְּעָלִים מְשַׁלְּמִין שְׁלֹשִׁים שְׁקָלִים, אֵין הַשּׁוֹר בִּסְקִילָה אֵין הַבְּעָלִים מְשַׁלְּמִין שְׁלֹשִׁים שְׁקָלִים.