Hello,
It depends on how you set up the Affirm account.
If the Affirm account is a liability account then you can use a journal entry as follows:
Payment Bank Account: Credit
Affirm Liability Account: Debit
Also, you can click the + sign in upper left corner and record an actual "Transfer" from the payment bank account to the Affirm account (provided it's a liability account). This will decrease the balance in the bank account and decrease the balance (owed) in the Affirm account.
