Below is query,
SELECT sum(isnull(Convert( float,Rec.Sell_Amt ),0))-sum(isnull(convert(float ,Pr.Rec_Amt ),0))as Amt,Pr.Level_Four_ID
FROM tbl_Receivable_Customer AS Rec
left outer join tbl_Received_Amount Pr
on Pr.Sell_ID=Rec.Sell_ID
where rec.Sell_Del is null and pr.Sell_Del is null
group by pr.Level_Four_ID