Updating the field


I want to update the voucher Field In SalesTable , i placed a voucher field in SalesTable and make a relation vth CustTrans

but i want when i check the Markup Trans in CustOpenTrans Form then that related voucher Should only update in My Sales Table Form in (Voucher Field). i tired alot but their is only one common field between SalesTable and CustTrans i.e CustAccount and Voucher is comming from CustTrans Table.

so please suggest the answer for this solution.