Here's an alternative:

IF EXISTS

(

SELECT * 

FROM Clients C

INNER JOIN InvoiceHeader I 

  ON I.ClientID=C.ClientID 

INNER JOIN Addresses A 

  ON A.ClientID=C.ClientID

WHERE I.InvoiceNr=@InvoiceNr 

AND A.PrimaryAddr=1

)

BEGIN

SELECT COALESCE(LTRIM(COALESCE(Salutation,'') + ' ' + FName + ' ' + LName), BusinessName) AS ClientName, A.* FROM Clients C

INNER JOIN InvoiceHeader I 

  ON I.ClientID=C.ClientID 

INNER JOIN Addresses A 

  ON A.ClientID=C.ClientID

WHERE I.InvoiceNr=@InvoiceNr 

AND A.PrimaryAddr=1

END

ELSE BEGIN

SELECT COALESCE(LTRIM(COALESCE(Salutation,'') + ' ' + FName + ' ' + LName), BusinessName) AS ClientName, A.*

FROM Clients C

INNER JOIN InvoiceHeader I 

  ON I.ClientID=C.ClientID 

INNER JOIN Addresses A 

  ON A.ClientID=C.ClientID

WHERE I.InvoiceNr=@InvoiceNr 

AND A.AddrTypeID=@BillingAddressID

END

HTH,

Jay