Employment Law Questions? Ask an Employment Lawyer.
They can only do so when the basis for deciding is made clear to the employees so that they understand what it is they are working for.
The agreement that your husband signed is almost certainly not an actual employment contract. Less than 1% of American's sign those. Most likely, it is just an employment agreement with a statement of terms which are "at will."
If it isn't, you can actually sue the employer for breach of contract, but the agreement has to actually state that the terms can't be altered without his consent.
Moving on to "at will" employment, the employer can alter that payment base at any time, but it has to be clear to the employee and forward looking. Your husband has to have the ability, each time, to understand his pay base and to make the person decision of whether or not he wants to work under that condition of employment.
If not, he needs to file a wage complaint against the employer with the state here: