Reference for Ketubot 209:13

הני מילי בטילא דלא מוכחא קרנא בטילא דמוכחא הוה שקיל דהוה תהי באמברא דחמרא ויהבי ליה זוזא

This applies only to a loss of work that cannot be proved, but Karna received [compensation for] loss of work that could be proved. For he was [regularly occupied in] smelling tests at a winery, and for this he was paid a fee.

Jastrow

Jastrow