Final Settlement on Hold

My Husband didn't get salary for Dec 2022 no intimation given on Hold of salary due to any reason. He waited for salary up to 15 Jan 2023 and also followed up may times over phone. HR and reporting manager also kept on telling they will check and revert. Finally he put down his resignation on 17th Jan 2023 and agreed to serve notice period also. However his reporting manager and HR had a con call and confirmed to relieve immediately same day without any notice period. During con call they informed that reason for holding salary is that My husband had given sales order to his colleague who was running a firm in his name other than being employed. 1) My husband not having any firm in his name / or running any business during his association with company. 2) Can they hold his F&F for this particular reason? Now they are asking him to provide list of activity. He has not done any activity which will incur loss or damage to his company. 3) He worked there for more than 10 years and his gratuity / Leave encashment / Dec 2022 full month and jan 2023 half month along with out of pocket expense (as he was into sales and marketing) on hold. Almost 3.5 to 4 lac is due for settlement. HR and his Manager both are avoiding calls and they don't intend to make any settlement. How do get this cleared...