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It's not illegal, as long as you have been made aware of the sales requirements in advance. By that, I mean that they can't look backwards in time and say "all those sales from the last quarter are now being treated under this new system we're starting today).
Unless you have a contract of employment that specifically states that your terms of employment cannot be altered without your consent, those terms can be altered at any time. The only restrictions are that you must be made aware of changes before they are implemented and you must still make more than minimum wage per hour.
Now, if the cut in pay is significant enough, on the order of 33% or more in total reduction in pay, that gives you the ability to give the employer an ultimatum that if the pay is not increased you'll be forced to leave. The purpose of this is to set up the ability to quit and yet still get unemployment.
Regrettably, that is your only recourse here other than simply accepting the new payment system.