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Unfortunately, if you are enrolled in Medicare part A, you are ineligible for an HSA. If the requirement of the plan is that there has to be an HSA attached to the plan, then your husband would be ineligible for the plan.
I don't know if the employer has a workaround or another type of plan to assist with this, or if you as a spouse would still be eligible for coverage.
Your husband would have to talk to his human resources department about the particulars of the plan.