Customers and Suppliers can be linked together to allow contra postings via either record type.
To create the contra relationship:
In a Customer or Supplier record
Go to Settings
Scroll down to the Contra Settings section at the bottom
Click New Contra
Enter or select the appropriate Supplier or Customer code (depending on which ledger you are currently in)
Save the changes
To post a contra transaction:
In either the Debtors or Creditors ledger
Expand Transactions and select Contra Adjustment
Search for the Customer, Supplier or both
Click on the pop over button (Blue ellipsis button) under the Customer Code or Supplier Code in question
Select Process Contra
Tick the appropriate transactions
amend the Allocated Value field to ensure they balance
Process to create the transaction, entering the required Analysis Code
Typically a Discount type analysis would be used for a Contra posting