בבא קמא סב:
"הנותן דינר זהב לאשה ואמר לה הזהרי בו של כסף הוא"
If one gives a gold coin to someone to watch, but tells him to be careful with it as it’s silver, if the שומר damages it בידים (throws it into the sea), he must reimburse him for the value of the gold. This is because the מפקיד may tell the שומר, “What reason did you have to damage my property?” But if the שומר was a פושע and it got damaged, he only has to pay the value of silver. This is because the שומר may claim he only accepted שמירה on a silver coin not gold. The [רצ"א, ד'] קצות החושן explains the difference between the פושע and the מזיק בידים: The חיובים of a שומר are חל, because when he accepts to watch an item, he is מקבל to be responsible for the safekeeping of the object he agreed to watch. So, when the שומר is told the object is silver, he only accepted שמירה and responsibility for silver. So, in our case if the שומר did not know it was gold, we view it as if he didn’t accept שמירה at all for this gold coin and he is like a stranger to it. This is why he is patur to pay for the value of the gold. Accordingly, even if while watching it the שומר realizes it is gold not silver, he would be פטור for a פשיעה, because he never accepted שמירה for gold. משא"כ the חיוב of a מזיק has nothing to do with his acceptance of paying for damages, his חיוב is a דין מן התורה and he is therefore חייב regardless of what he thought the item was. The [חו"מ ס' רצא, ז'] סמ"ע explains, true the שומר is חייב if he was מזיק בידים, but the מפקיד must bring proof that it was gold not silver. The ש"ך explains that although normally the מפקיד would be believed with a שבועה, here he wouldn’t be believed because he already said it was silver, he must therefore bring actual proof that it was gold.
The [נדרים כה.] בן יהוידע quotes the Gemara (קניא דרבא) where רבא made a לוה swear that he paid back his מלוה. The לוה hollowed out his cane and put the money he owed the מלוה in the cane. When he arrived in ב"ד he had to hold the ס"ת to make his שבועה so he asked the מלוה to hold his cane for him. He then swore that he gave the money in question to the מלוה. The מלוה got so angry, that he broke the cane and all the money came spilling out. The בן יהוידע klers, if this took place near a river and the מלוה threw the cane into the river in a rage, if he would be considered a שומר חנם, based on our gemara since he damaged it בידים, would he be חייב for the money hidden in the cane cancelling out the loan?
The בית הלוי על התורה explains the pasuk: "כי נבלה עשה בישראל וכן לא יעשה" with our Gemara. The שבטים said that since it was אסור for שכם to do what he did, even to a non בת ישראל (either because of the גדרים accepted upon themselves or because it was בעל כרחה), he is now considered a מזיק בידים. So, like in our gemara a מזיק בידים can’t use the excuse he wasn’t aware of the value of the object he damaged, so he could be told what right do you have to damage my property, so too, שכם can’t claim he didn’t know she was a בת יעקב, and should not be punished severely, because they can respond with, “What right did you have to be מטמא דינה?”