I agree, a contract is binding once it has been signed. What you may want to do to somewhat appease your new company is to ask them to provide you with the official clause/Article in the "new Labour Law" that the company is referring to(which they claim allows them to decrease your salary after singing the contract). You could also suggest that both parties go to the Labour Department with the signed contract in hand to get a ruling from the Labour Department on the company's request. My guess is that the company will cease to ask for this increase with this subtle threat to go to the Labour Department.