Final answer:
The debit/credit memorandum is the source document common to both the sales and purchasing processes, used to rectify discrepancies in transactions.
Step-by-step explanation:
The source document(s) common to both the sales and the purchasing processes is the debit/credit memorandum. This document is used in both processes to rectify any discrepancies that may occur, such as incorrect billing or returns. Financial statement information is typically a summary report rather than a source document used in transactions, while discrepancy reports and bad debt reports tend to be more specific to account management and are not common transactional documents.