What the owner did was perhaps despicable, but probably not duress. Assuming that you didn't have a contract already that protected your position there, he could have terminated you anyway. This new agreement, while being forced upon you there is no literal gun to your head. Figurative gun perhaps, but just because it is a bad deal for you doesn't mean that it is duress.
I suggest that you look on Avvo for a local business attorney and have them review the agreement before you sign. If you have already signed, take it to them anyway. They may have a different opinion once they have all of the facts.
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Duress is very misunderstood by non-attorneys. A tough, one-sided, take it or leave it, agreement does not present "duress". You may refuse to sign and work elsewhere.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
It is not duress if you have been given two days to consider whether you want to sign a contract which contains unreasonable terms. You have a choice. You even have the time to seek legal advice, although getting free advice on the internet may not be the best place to get that advice.
This business had decided to impose terms upon its tenants that are favorable to it and unfair to you. If you do not want to agree to those terms, it is time to take your business to another salon. It is done all the time. You can make a counter-offer if you like. If the salon owner does not want to negotiate then it would be bad business for you to continue working under the new arrangement.
Now, if there are terms in the agreement that are unlawful, or if this salon owner has misclassified you as an independent contractor when you are really an employee in the eyes of the law, that is another matter. But there is no way anyone can tell from these facts if anything about the contract or your business relationship is unlawful, without reviewing the contract and discussing the facts. For this, you need to consult with an attorney.
My free advice: do not sign a contract you cannot live with or agree to. It will be extremely difficult to get out of it by claiming you were coerced into signing it.
They say you get what you pay for, and this response is free, so take it for what it is worth. This is my opinion based on very limited information. My opinion should not be taken as legal advice. For true advice, we would require a confidential consultation where I would ask you questions and get your complete story. This is a public forum, so remember, nothing here is confidential. Nor am I your attorney. I do not know who you are and you have not hired me to provide any legal service. To do so would require us to meet and sign written retainer agreement. My responses are intended for general information only.
The owner of the salon and you are free to negotiate whatever terms you each want in reaching an agreement as long as they are not illegal or in contradiction to some public policy. There may be terms you do not like, but the owner is free to include those terms. If you don't like the terms and are not willing to accept them, then you are free to go elsewhere to work.
The circumstances you describe are not sufficient to constitute duress. Since the owner has already told you that he will not negotiate, your choices are to sign the agreement and keep working at the salon, or find somewhere else to work.
You might want to take the agreement to a local attorney for a consultation, just to be sure there is nothing illegal elsewhere in it.