As the PRTB have told you, you cannot terminate the fixed term lease (unless you have a suitable break clause, which you probably don't). Therefore, you cannot serve a valid Notice of termination which will expire before a fixed term lease expires. IMO, you have misunderstood what the PRTB have stated. You can serve the Notice today as long as it expires, at the earliest, the first day of the next lease period. In this case, the tenancy will be a Part 4 tenancy and under Part 4 rules you may evict a tenant for reason of the sale. However, do pay attention to ALL the requirements of a NoT for the sale of a rented property as a ground for termination of a lease.