New Landlord - Deposit Disbursement question

[Mario F.]

Hello, I just purchased a fourplex in Inglewood CA.  I have a renter leaving this weekend.  Once I am done with his walkthrough on Saturday, I'lll need to give him his deposit back.  Do I give him it at that point? Or should I wait and send it via mail? 

 All of the tenants have been living in these units for over a decade and so the question I have is regarding the interest accumulated on these deposits.  The seller has not given me any interest on the deposits.  What do I do about this?  Am I required to calculate the interest accumulated over the past 15 years of their residency? Or do I get a pass since I'm the new owner?  All I have are the old lease agreements which state the deposit amounts they put in when signing. I assumed that it would be OK to just give them that amount but my brother is telling me I'll need to send them the deposits plug interest accumulated over the years. Any thoughts? 

thanks

mario