Credit card installment billings are automatic, but sometimes a member wants to 'pre-pay'. In that case, go to the member's cash account and add a DEBIT for the amount to be paid (memo = pre-paid 3rd installment or something like that), click Bill For Cash. NEXT, add a CREDIT for that same amount. This will put a positive balance in the member's cash account and leave it there.
When the next installment billing runs, it will pull the funds from the member's cash account.