Contact the tenant directly. If no contact number, go there to talk face to face nicely. Tell them that u guys are the real owner and want to sell that house.

I'm afraid that guy X not even pass the ending agreeement prepared by u guys to the tenant.

By law, owner can just sell the house and give example 3 months notice to the existing tenant to move out. Or, sometimes, the tenant themself willing to purchase the house.