Reference for Bava Kamma 86:2
גבי עבד שאינו משלם קנס ע"פ עצמו והיכי דמי דאי אתו עדים ואסהידו ביה דקטל ולא ידעי אי תם הוה אי מועד הוה ואמר מריה מועד הוא לא משתלם קנס ע"פ עצמו היכא דליכא עדים לא משלם דמים
— that [we say] where witnesses are not at all available payment will be made for the actual value [of the loss]. [Whereas] in the case of a slave where the fixed fine could never be paid through the defendant's own admission — since even where witnesses appear and testify to the ox having killed [a slave], without knowing whether it had still been <i>Tam</i> or already <i>Mu'ad</i>, and the owner admits that it had already been <i>Mu'ad</i>, no fine would be paid — [we say] where no witnesses at all are available there will be no payment even for the amount of the value [of the loss].