Gracias Iván, a lo mejor no me expliqué de la mejor forma, ya resolví:
SELECT DISTINCT
CASE WHEN T0.[CardName] = 'C00001' THEN T0.[DocNum] ELSE T0.[DocNum] END'# de Fact.',
CASE WHEN T0.[CardName] = 'C00001' THEN T0.[CardCode] ELSE T0.[CardCode] END'Código',
CASE WHEN T0.[CardName] = 'C00001' THEN T3.[U_Proveedor] ELSE T0.[CardName] END'Proveedor',
CASE WHEN T0.[CardName] = 'C00001' THEN T3.[U_RUC] ELSE T2.[VatIdUnCmp] END'RUC',
CASE WHEN T0.[CardName] = 'C00001' THEN T3.[U_FACTURA] ELSE T0.[NumAtCard] END'# Fact. Prov.',
CASE WHEN T0.[CardName] = 'C00001' THEN T3.[U_Coment] ELSE T0.[Comments] END'Comentario',
CASE WHEN T0.[CardName] = 'C00001' THEN T3.[U_RET] ELSE T1.[Rate] END'Tarifa',
CASE WHEN T0.[CardName] = 'C00001' THEN T3.[LineTotal] ELSE T1.[TaxbleAmnt] END'Importe Sujeto',
CASE WHEN T0.[CardName] = 'C00001' THEN ((CONVERT(NUMERIC(18,2), T3.[U_RET]))/100)*T3.[LineTotal] ELSE T1.[WTAmnt] END'Importe Retenido'
FROM
OPCH T0
INNER JOIN PCH5 T1 ON T0.[DocEntry] = T1.[AbsEntry]
INNER JOIN OCRD T2 ON T0.[CardCode] = T2.[CardCode]
INNER JOIN PCH1 T3 ON T0.[DocEntry] = T3.[DocEntry]
WHERE
T0.[DocDate] >=[%0] AND T0.[DocDate] <=[%1]
Post Cerrado