I'm not aware of a minimum amount - but I'm willing to be corrected. If it was going to be a hassle though I'd probably write it off. The other thing you could do is to write to the tenants before they leave telling them what they must do to ensure they receive their full deposit back i.e. leave the property as per the inventory, clean the oven etc and add the return of parking permits to the list. You could even state in the letter how much the deduction would be if they dont return them, then there can be no argument.