The property belongs to the owner /landlord
Without his consent/noc it cannot be sold
Otherwise it would amount to illegal subletting
So the tenancy has to be transferred to the purchaser
However since the whereabouts of the landlord are not known it would not be possible to transfer tenancy to buyer
And there would be few buyers who would be interested to buy without the tenancy being transferred to the buyer's name
So your FIL can form a partnership firm having himself and one other partner
The partnership firm can be registered at the address of the flour mill
Your father can keep a very small stake in that firm , say 1% and that too as a non executive partner
The other majority partner would be the buyer who would in essence own the firm
There can also be a side agreement in which it will be agreed that your FIL will not stake any claim in the property by virtue of being its 1% shareholder
As a commercial tenanted property can be used by a partnership firm formed by the tenant, that's the only way out I can see your FIL exiting from the property and without incurring any liability on the prospective purchaser if he were to purchase the tenancy rights without transfer of tenancy
But the rent receipt will continue to be issued in the name of your FIL
However he can give a PoA to the buyer
So the buyer will have to compromise on the title which he would get since ideally there has to be a registered triparty tenancy agreement between outgoing tenant, incoming tenant and landlord
I dont think capital gains tax on sale would be an issue since generally tenancy transactions are done in hard cash
But few buyers do prefer to do payments by banking channels
So there will be not many buyers who would be interested given the above complications
But those who are interested can certainly explore the above way out
Also the purpose of the use would be changed by the buyer from flour mill to some other business. If there is no restriction in the tenancy terms regarding change of user then that may also not be an issue
All in all this transaction would be a risky proposition for any buyer