Taberî first explains that the tâ letter in the expression 'تالله' (tallahi) is originally an oath wâw, which was transformed into tâ because Arabs frequently used it in oaths. He notes that this transformation occurred only with the name 'Allah,' and not with other names like Rahmân, Rahîm, or other things sworn by. He then reports that the brothers' statement indicates they knew 'they did not come to rebel against Allah (steal/commit corruption) in your lands,' and explains how they dared make this claim, basing it on the fact that they knew they had returned the provisions and had not wronged anyone during their journey.

وإنما جعلت تاء لكثرة ما جرى على ألسن العرب في الأيمان في قولهم «والله» ، فخصت في هذه الكلمة بأن قلبت تاء ومن قال ذلك في اسم الله، فقال: «تالله» لم يقل تالرحمن، وتالرحيم، ولا مع شيء من أسماء الله

According to Zemahşerî in this text, the brothers are expressing that they have never been characterized by theft and that this crime is something contrary to and inappropriate for their condition/state. That is, theft is something that contradicts their known moral character.

وما كنا قط نوصف بالسرقة وهي منافية لحالنا

Kurtubî reports the tradition that Ya'kub's children did not wrong anyone, caused no harm to anyone's crops, and bound muzzles on their camels' mouths so they would not enter and damage others' fields. He also mentions that they returned the capital/property that was with them (forgotten), and points to the conclusion that one who returns what he finds cannot be a thief.

يروى أنهم كانوا لا ينزلون على أحد ظلما ولا يرعون زرع أحد وأنهم جمعوا على أفواه إبلهم الأكمة لئلا تعيث في زروع الناس

According to this exegete, it is explained that from the time the people knew these individuals, through the good conduct and behavior they witnessed from them, they definitively and firmly knew and understood that their character did not warrant this (evil) attribute.

أي لقد تحققتم وعلمتم منذ عرفتمونا لأنهم شاهدوا منهم سيرة حسنة أنا أي ليست سجايانا تقتضي هذه الصفة

According to Beydâvî, in these verses it is explained that the king's officials announced that they had lost the sikka/water vessel (savâ') and promised a camel's load of reward to whoever found it, informing them that they would be guarantors of this reward to whoever brings it. Yusuf's brothers, for their part, swore by the name of Allah that they did not come to commit corruption and that they were not thieves, supporting these claims with the trustworthiness and integrity they had shown in their previous visit and in their comings and goings with the king (such as returning the price of provisions to their loads and binding the animals' mouths so they would not damage others' crops).

وفيه دليل على جواز الجعالة وضمان الجعل قبل تمام العمل. قالوا تالله قسم فيه معنى التعجب، التاء بدل من الباء مختصة باسم الله تعالى