daf bit: Bava Metzia 40

[cellio]

The mishna teaches: if a man entrusts produce (it speaks specifically of grains) to another for safekeeping, the trustee is allowed to deduct a fixed percentage for loss (e.g. due to mice) when he returns it. A tanna teaches that this is true only if the trustee mixed the deposit with his own grain; if he did not, and instead placed the other's grain in a separate pile, then upon return he simply points to the pile and says "here is yours". (40a)

(It is a fixed proportion of the deposit rather than a proportional share of the actual loss because the latter is too hard to measure, since the trustee would be taking grain from the store for his own use during this time.)