How to update Tr code FB02 and update the field reference key1 (bseg-xref

Question

Can any body
help me how to update   Tr code   fb02
and  update the field  reference  key1 (bseg-xref1)  with the interest amount
calculated.
Below i have  written My  program  flow logic
The program should read the open items i.e debit items in the customer account,
Based on the  invoice date and customer payment terms  calculate the due date and then the  interest should be  calculated on that item based on the due date.
Use the  transaction  fb02  and update the field  reference  key 1 (BSEG-xref1)
with the interest amount calculated
Only  consider  customers whose  interest indicator  knb1-vzskz = 'vk'.
Interest is to be  calculated for every  30  days
at the rate of  1% .please see example below.
Document date is 01.01.2007
for amount  1000SGD   payment  term  7  days
It becomes  due  on  08.01.2007
until  06.02.2007 you should not  consider  for calculation of interest
SGD  on   7.02.2007 run you can consider this  item for interest calculation

Answer 1

Hi Robert,
Thanks for answering.
I am looking at Ref 2 should flow automatically to clearing document from the accounitng document for billing.
Ex: Billing document is created and accounted. Ref 2 is AB
Now when receipt is accounted for this biling document this AB should flow automatically to Ref 2 field.
Thanks
NNS.