I have 3.5k in security (plus some interest. 

security deposit are the tenants, not the owners.

The security deposit would be applied to any monies owed and a letter would be sent within 30 days to the tenant. 

I own a 17 unit apartment building near Minneapolis built in the 60s and wanting to implement wifi camera system and modernize the access panel (wifi managed).Any vendor recommendations?

What are the ways to secure remaining money for this deal.

@Mike Dumont so apart from some correspondence detailing how the funds would be handled, I would do the following.Colorado law gives the landlord 30 days to return a security deposit.

Legal Mistakes Can Lead to Lawsuits or Tenant ClaimsTexas has landlord-tenant laws on security deposits, repairs, evictions, and fair housing compliance.If you mishandle security deposits, maintenance requests, or notices, your friend could get sued or be forced to compensate tenants.Evictions require proper legal procedures.

You will receive the security deposit in Checking but then transfer it to Savings.

Installed 3 brand new hvac systems in each unit, undated kitchen and bathroom and added security cameras on exterior of property.

I'm curious what others have found useful for their outdoor camera setup and lock systems (WiFi...?)