This should work as long as you don't have multiple rows being returned from that subquery

SELECT dbo.nomfil.nomcode ,

ISNULL(calc.val, 0) AS ytdper ,

calc.costcentre

FROM dbo.nomfil

LEFT OUTER JOIN ( SELECT CASE WHEN nomfil_1.type IN ( 3, 4 )

THEN nomtrn.nomcode

ELSE ( SELECT nomcode

FROM ctlfil

WHERE recid = 24

)

END AS nomcode ,

SUM(dbo.nomtrn.val * ( 1 - 2 * dbo.nomtrn.crddbt )) AS val ,

dbo.nomtrn.costcentre

FROM dbo.nomtrn

INNER JOIN dbo.nomfil AS nomfil_1 ON dbo.nomtrn.nomcode = nomfil_1.nomcode

WHERE ( dbo.nomtrn.period < ( SELECT

glpernum / 100 * 100 AS Expr1

FROM

dbo.perfil

) )

GROUP BY CASE WHEN nomfil_1.type IN ( 3, 4 )

THEN nomtrn.nomcode

ELSE ( SELECT nomcode

FROM ctlfil

WHERE recid = 24

)

END ,

dbo.nomtrn.costcentre

) AS calc ON calc.nomcode = dbo.nomfil.nomcode