For your existing primary, you will need to have it leased with the security deposit and first month's rent collected.

@Michael Anderson If the property had been sold, it would make sense to formally assign the lease and deposit to the new owner and memorialize an agreement to collect and adjust rents after settlement, indemnify the other party for things arising during their respective ownership periods, etc. 

I think using a deposit model where they can get some those prepaid rate back upon closing is a good way to induce commitment.   

@Danny RandolphYeah, I’m currently in the construction field and we usually get a 20-40% deposit, then get draws after certain stages/inspections have been passed

The security deposit should never be used for anything during the lease.

Another reason to send it by mail is in many cases you will be deducting damages from deposit.  

The vast majority of lawsuits against Landlords are for wrongful eviction, security deposit disputes, and Fair Housing Violations.

Often there is a process to withhold rent and deposit it with the court until the issue is resolved.

Cash flow is the amount left over that you deposit into your real estate-only account. 

I would increase their deposit by that amount to ensure you have enough to cover an empty tank and a month of lost rent.