If one deposits produce with his fellow, even if it is becoming ruined due to mice or spoilage, the custodian may not sell it.

One reason is that a person prefers a measure of his own produce - for which he toiled - to nine measures of his fellow's produce which can be bought. That is an exaggeration, but if it is not spoiling faster than normal, it should not be sold.

Another explanation is that he may have designated it as tithes for his other produce, and he may come to eat non-tithed produce. Knowing that the produce is not sold, the owner is even more likely to do this designation.