You need to contact an attorney directly--an online forum is not a good vehicle for discussing contracts that have not been reviewed--the first thing an attorney will need to see when determining your rights under the contract is the contract itself.
READ THIS BEFORE CALLING OR EMAILING ME: I am licensed to practice before the state and federal courts in Virginia. We have not established an attorney-client relationship unless we have a signed representation agreement and you have paid me. I am providing educational instruction only--not legal advice. You should speak with an attorney to whom you have provided all the facts, before you take steps that may impact your legal rights. I am not obligated to answer subsequent emails or phone calls unless you have hired me. I wish you the best of luck with your situation.
Your client attempted to unilaterally change the terms of your agreement. This type of contract modification, or "change order," requires new consideration to be valid. The fact that you said, "OK, then let's modify it in order for me to get paid for what I've already done" is not really a mutual agreement to modify the contract. Without your client giving up something new in value to support the modification (for example, agreeing to a higher hourly rate for what work you do perform, or requiring more prompt payment upon receipt of the invoice), there is no contract modification. I believe that you are entitled to be paid under the original terms of the agreement. At this point, I would not perform new work. I would send a strongly worded letter. I would then consider how much is at stake, and whether litigation is actually worth it. Mediation may be the way to go, but many personality types refuse mediation until forced into it through the litigation process. If you sue for less than $10,000 you can recover your attorney fees in Washington. If you are a corporation, you cannot represent yourself, but must hire an attorney (unless you are the sole shareholder). If you represent yourself, and go the small claims route, service of the summons and complaint are very important. Follow those instructions very carefully.
Without knowing all of the details, reviewing documents, and interviewing witnesses, no person should assume that this Answer constitutes specific legal advice for any specific legal situation. No attorney-client relationship is created by posting general legal responses on this site.
The answer to this question depends a great deal on the terms of the actual contract itself. You should consider meeting with a litigation attorney in your area to review the contract, and to evaluate your options on whether and how to proceed. Assuming that you have a good claim for payment under the contract, the amount of money at issue will also guide your decision making, as a relatively modest value claim may not be worth the trouble of a full-fledged lawsuit. Small claims court could be an option, but engaging a lawyer can also make sense in certain lower-value cases; Ms. Young rightly points out that there are statutory mechanisms in Washington that allow recovery of attorneys' fees in cases worth less than $10,000. At the very least, you should be able to secure a free consultation with an attorney to review the contract and discuss your options.
Benjamin Nivison is an attorney licensed to practice law in Washington State. This communication does not create an attorney-client relationship between you and Mr. Nivison, nor does it constitute specific advice for your particular legal matter. The information provided in this communication is for general reference and informational purposes only, and therefore should not be relied on as legal advice. Legal issues are by their nature complex, and any person with a legal question should fully consult with a qualified attorney.