PAYMENTS looks like this:
 BEN12, FLY, 8.00
 BEN12, CAR, 2.00
 MKE10, FLY, 3.00

 and Applicaitons needs to look like this:
 BEN12, 1234-10,5.00
 BEN12, 1234-11,3.00
 BEN12, 2234-20,2.00
 MKE10, 1234-13,3.00

`declare @currentCust varchar(20) =''
 declare @LastCust varchar(20) =''
 declare @AmtBal decimal (5,2) ='50'
 declare @applyBal decimal (5,2) ='0'
 declare @CurrentDoc varchar(20) =''
 declare @LastDoc varchar(20) =''
 declare @CurrentCod varchar(20) =''
 declare @LastCod varchar(20) =''

 begin
 UPDATE USER_PRD_APPLY
 set CODE = DOCS.DED,
 @CurrentDoc = DOCS.doc_NO,
 @currentCust = DOCS.CUST_NO,
 @CurrentCod = DOCS.DED,
 CUST_ID = DOCS.CUST_NO,
 @AmtBal = case when @currentCust = @LastCust THEN @AmtBal ELSE RCV.AMT END,
 @applyBal = case
        WHEN DOCS.DED = '608' and DOCS.balance <= @AmtBal
        THEN DOCS.balance
        when DOCS.DED = '608' and DOCS.balance > @AmtBal
        THEN @AmtBal
 @LastCust= DOCS.CUST_NO,
 @AmtBal = @AmtBal- @applyBal

 from DOCUMENTS DOCS left join RECEIVE RCV
 on DOCS.CUST_NO = RCV.CUST_ID and DOCS.DED = RCV.CODE

 end