The purpose of the debtors journal is to record all credit sales of the business. Credit sales occurs when the business sells goods now, but will only receive payment in the future. As money is owed to the business an asset debtors control is created. As more money is owed the asset debtors control increase and therefore have to be debited. The contra entry is sales. According to the double entry principle therefore sales have to be credited with debtors control as contra entry. As the sales column total is involved the posting occurs at month end.
The debtors journal also contains the cost of sales column to keep track of profit made on each item sold for credit. Before selling the trading stock it was the possession of the business and hence an asset. When the sale occurs therefore the asset trading stock decrease. As assets decrease trading stock have to be credited. The contra entry is cost of sales. According to double entry principle the cost of sales account should be debited with trading stock as contra entry. Since cost of sales is a column total the posting occurs at month end.