What Calif legal statute can I refer to in my demand letter to my commer landlord to return my security dep?

I gave 30 day notice and moved out of my commercial rental space. There were no damages and the property was left clean. The landlord sent me a check for $250 instead of $500. I paid a $500 deposit in 2001. She keeps using the excuse that she has not had time to check her files to confirm that I actually paid a $500 deposit. Fortunately I kept a copy of my commercial lease agreement which states that I did leave a $500 deposit. Its been over 40 days and she will not return my calls. She is very unorganized and does not keep good records. I want to send her a demand letter requesting the balance of my deposit but need to know what legal statute I can refer to in my demand letter to her. I am in california. Thank you