Reference for Bava Kamma 231:8
ואי אשמעינן סיפא הכא הוא דבסתמא אין לו אלא שכרו משום דממילא אבל התם דבידים אימא אפילו בסתמא יהיב ליה דמי כולה צריכא
This shows that he is entitled to say, 'I was merely jesting with you'? Why then also here should he not be entitled to say to him, 'I was merely jesting with you'? — The comparison is rather with the case dealt with in the concluding clause: But if he said to him, 'Take this <i>denar</i> as your fee for ferrying me across,' he would have to pay him the sum stipulated in full. But why this difference between the case in the first clause and that in the second clause? — Said Rami b. Hama: [In the second clause] the other party was a fisher catching fishes from the sea in which case he can surely say to him, 'You caused me to lose fish amounting in value to a <i>zuz</i>.'<span class="x" onmousemove="('comment',' I.e., the denar you offered me; in the case in the Mishnah the same argument holds good, hence the same ruling. ');"><sup>8</sup></span>