Effectively the employer reduced your husbands salary without and discussion or agreement. I am not sure of the legality of this but surely it is not above board. You can rest assured that if the tax rate on your husbands salary increased the employer would be looking to renegotiate. I would approach the employer and asked to be compensated. If you do not succeed with this look into possibly perusing legal options.
Get your husband to negotiate a gross salary going forward.
Get your husband to negotiate a gross salary going forward.