As well as the negative value in purchases to cancel out the expense, in the current account sheet there would be a negative number entered in the Payments section using the code CR (Creditors).
This should cancel out the original purchase which would have included:
A positive entry in Purchases
A positive enter in Current Account as a Payment to Creditors (CR)
(This explanation assumes a transaction going through the current account, it could equally have been a cash or credit card transaction.)
If the item was never actually paid for there would be no need to enter the bank transactions.