They are allowed to negotiate changes to an employees contract. Your husband can either accept the offer or reject it. If he rejects it the employer may possibly terminate his contract. If this happens your husband has the option of claiming unfair dismissal - though the success of this would depend on a number of factors (such as whether the company wanted to make the changes for sound financial reasons i.e. they can't afford to pay the old salaries any longer). If the company can show it had good reason for wanting to change the salary terms, and if most of the employees accept the new terms, your husband's claim for unfair dismissal would probably fail.
Any changes to the contract should be done with consultation though.