פירוש על עבודה זרה 30:12

You might think that a Samaritan will definitely not do teshuvah and stop selling weapons to non-Jews, but a Jew known to sell weapons (or other contraband) to non-Jews might stop doing so, and therefore it is permitted to sell weapons to non-Jews known to sell to idolaters. But there is yet another source in which R. Nahman explicitly states that any item that may not be sold to an idolater, may also not be sold to a Jew suspected of selling to an idolater. This is a clear rejection of Rabbah.

Rabbah tries very hard to get the donkey back but he failed. Poor Rabbah, but I guess he at least got some good exercise.