If a Clients asks for their data to be deleted, but they are involved in a current business transaction, what will happen to these transactions?

When you are asked to delete an individual's personal data (from the Record, NOT the Grid), you will need to check if they have a pending or accepted offer (Applicant), if they are a potential or current tenant (Applicant, or if they are selling (Vendor) or renting (Landlord) a property with you.  You would be advised to action changes in relation to this ongoing business because once you proceed with the deletion, you will not be able to identify the applicant, Vendor or Landlord on the Offer, Tenancy, Property, Viewing etc.