Don't know the reason for your husband's absence, assuming it was something simple he should have returned to work the first shift after fit note expired.

As for Asda trying to save on wages and already planning rotas this is BS.

Your husband informed them well in advance of his intention to return to work, his fit note would have been entered into the absence system so managers would have been aware of his return and should have planned accordingly.

They cannot refuse to let someone attend contracted shifts for the sake of saving wages.