This is probably not an employment law issue. it sounds like what you have is a breach of contract. You can sue for breach whether the breach is by your employer or some third party. It is possible that you may be able to make some sort of a wage claim based upon the fact that your employer asked you to perform these services, but the fact that you have a contract for this work probably will work against you in that regard.
Speak with a Washington employment or general civil litigation attorney.
I would agree that is sounds like this is unrelated to your employment. You mention an original contract. Was this contract in writing or was it an oral contract. Was the totally different contract signed by both parties. I would say generally this issues passes outside of employment unless the employer somehow made the sidework a condition of employment. I would recommend the smalls claims court. No attorney needed if you do that.
I agree with both previous answers and you could file a complaint with L&I or go to small claims court. But you will have to prove the case and explain the facts much more clearly and show the evidence of what you claim.
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