Al parecer, haciendo el siguiente 'Cast', funciona :
Código SQL
[-](Select Sum((INV3.AMOUNT - ROUND(INV3.AMOUNT * Coalesce(INV3.DISCOUNT,0) / 100, 2)) +
Round(((INV3.AMOUNT - ROUND(INV3.AMOUNT * Coalesce(INV3.DISCOUNT,0) / 100, 2)) *
Cast(IIF(INV3.VAT_TYPE = 1, FX.VAT_TYPE_1,
IIF(INV3.VAT_TYPE = 2, FX.VAT_TYPE_2,
IIF(INV3.VAT_TYPE = 3, FX.VAT_TYPE_3, 0))) as Float) / 100),2))
from INVOICES INV3
left join FIXEDVALUES FX
on INV3.BUILDINGNO = FX.BUILDINGNO
and ((INV3.INVOICEDATE is not Null and INV3.INVOICEDATE between FX.DATEFROM and FX.DATETO) or
(INV3.INVOICEDATE is Null and 'TODAY' between FX.DATEFROM and FX.DATETO))
where INV3.INVOICENO = INV.INVOICENO
and (((INV.INVOICESERIE is null) and (INV3.INVOICESERIE is null)) or
((INV.INVOICESERIE is not null) and (INV3.INVOICESERIE = INV.INVOICESERIE)))
and INV3.DEBIT_CREDIT = 1
and INV3.VAT_INCLUDED = 0) as TOT_DEBIT_VAT_EXCL,
Saludos !
