As I see this (I should read this topic for another 3-4 times to fully understand the need of wtanksleyjr) – you should make some RESERVATION of your funds. I would open another “virtual” account (named RESERVATION, for example) and transfer funds onto it! Also, I would make another transfer (back) on the exact date of credit pay-off. You could look at it as some savings account.
I tried this — it doesn’t work, because it throws off all of the CalendarBudget bank reconciliation.
For a future bill with a known date there’s another option — simply manually update the future bill. As long as you remember to scroll a couple months ahead to look for negative cash balances you’ll be OK. It’s a pity that’s a lot of manual work if you use a credit card a lot, but it’s better than nothing.
And odds are that your credit bill will be pretty much the same every month, so perhaps you can get away with only entering a guess, and then updating the guess when the bill arrives.