sp_executesql

SET @sql = 'SELECT @rpta = SUM(importe)
FROM   ' + @ls_tabla + '
WHERE  concepto = (SELECT valor FROM FPV5_EC_parametros_maestro WHERE id = 150)
       AND trabajador IN (SELECT DISTINCT(tra_cod)
                          FROM   #trabajadores)'


EXEC sp_executesql @sql,
     N'@rpta float output',
     @rpta = @hor_tra OUTPUT


SET @hor_tra = COALESCE(@hor_tra, 0)