As Arthur has said, your husband is required by law to declare any change of circumstances to any and all agencies who are paying benefits to him. Otherwise he may be committing fraud.

These agencies will then revise what (if anything) is payable in benefits, having taken into account your earnings, plus any other changes.

Even if there has been an over-payment by them, they will give your partner time to repay it. The most important thing is that he reports the changes to them NOW, to avoid getting into trouble.