Briefly

If you take some money under the term "security deposit", you are required to return the whole amount and void the lease. 

However, this deposit is paid under the term "holding deposit", then you can withold the "holding desposit", and when you find the next tenant, deduct your actual loss from the holding deposit and return the rest. 

Your actual loss from holding the property and the lost rent starting original move-in date to the starting date of next tenant's lease.   The holding period from original signing date to now has no actual loss of rent because you agreed to keep your property vacant.