Go to G/L > Account Maintenance
Create a new normal account under Expenses (or other Account Type)
This account is a temporary account for A/R and A/P contra only. This account will be zero after contra.

Go to Tools > Options > G/L > Defaults Accounts
Select the account created in A/R & A/P Contra Temporary Account.

Go to A/R > A/R and A/P Contra Entry