Answer
For a contra (same party is both customer and supplier), credit the sales ledger control account and debit the purchases ledger control account with the offset amount, reducing both. The same entries are mirrored in the two personal accounts. This reflects settling mutual balances net.
Contra 300:
Dr Purchases Ledger Control 300
Cr Sales Ledger Control 300Real-world example Offsetting a mutual balance reduces both the receivables and payables control accounts by the contra amount.