Here's the WA security deposit law:

http://app.leg.wa.gov/RCW/default.aspx?cite=59.18.253

Here's the WA small claims information:

http://www.courts.wa.gov/newsinfo/re...sinfo_jury.scc

You don't need a lawyer to sue in small claims court. When you do he will have to present his receipts and justify how and why he applied your deposit.