Got it. Thank you for the additional information.
You do have 21 days to return the security deposit, and you might be responsible for the damage the mold caused IF you were aware of a problem (i.e., tenants notified you) and you failed to remedy the problem within a reasonable amount of time, and if the tenants had no reason to know that storing the shoes in the closet would result in damage until the damage was done.
If it's possible to ascertain the amount of the deposit sooner, rather than later, it may keep it difficult situation from getting worse. It would be a good idea to get the former tenants to sign off on any claims against you when the security deposit is returned, but you cannot withhold the deposit. In any event a box of shoes does not seem worth it for even a small claims lawsuit. The key here is to avoid a potential habitability lawsuit.
The former tenants cannot do anything to you if you wait to return the deposit as long as the accounting and deposit are handled before the deadline. As mentioned, you just don't want this to turn into a habitability case.
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