you can try something like:
ALTER procedure [dbo].[SP_Chk_Purchase_Bal_By_SupID]
@supid int
as begin
declare @amt_paid float
select @amt_paid = sum(rf.Paid_amt) from tbl_pur_Child as rf join
tbl_Purchase as pu on
rf.Purchase_ID=pu.Purchase_id where Supplier_ID=@supid
select
PU_Ref ,
Original_Amt,
(Original_Amt- @amt_paid)
from tbl_Purchase where Supplier_ID=@supid
end