Kommentar zu Bereschit 44:9

אשר ימצא אתו מעבדיך ומת, "whichever of your servants it will be found with shall die, etc." Gentiles are subject to the death penalty for theft (Sanhedrin 54); the brothers offered themselves as slaves in addition

אשר ימצא אתו מעבדיך ומת, “whoever of your servants it will be found with shall die.” They were so certain that none of them had stolen it that they could safely make such a statement as decreeing death on the thief if he were one of them. However, afterwards, when the goblet had been discovered in the travel bag of Binyamin, they no longer mentioned the death penalty but offered themselves collectively as slaves to Joseph.