Thanks everybody but I found the result with this query
from b in db.PurchaseOrderDetails
select new
{
b.RawMeterialMaster.RowMeterialName,
PurOrderDetailId = b.PurOrderDetailId,
PurchaseOrderId = b.PurchaseOrderId,
RawMetrialId = b.RawMetrialId,
DueOn = b.DueOn,
Quantity = b.Quantity,
ItemRate = b.ItemRate,
MetrialUnitsId = b.MetrialUnitsId,
Remark = b.Remark,
RowUnitName = b.RawMeterialMaster.RawUnitMaster.RowUnitName,
ReOrderQuantity = (decimal?)b.RawMeterialMaster.ReOrderQuantity,
Remain = ((System.Decimal?)b.Quantity -
(from a in db.PurchaseDetails
where
a.RawMetId == b.RawMetrialId &&
a.PurchaseMaster.PurchaseOrderId == b.PurchaseOrderId
select new
{
a.Quantity
}).Sum(p => p.Quantity) ?? (System.Decimal?)b.Quantity)
};