What happens to the scheduled payment if I don't have enough money to cover it ?

I have made mistakes on my account, but have been able to cancel them and make another payment if possible.

See if you can call your bank and ask to have it cancelled, which I have done with my Chase account after they changed it and it drove me crazy and could not find payments/records like before, or kept kicking me out from one part to another. Or, in some, they may have a place to ask questions, that will tell you how, or a chat on the account.

If, can't tonight, if you call at 8:00 AM sharp, might be able to cancel, since many times will say w/drawn at 5pm.

It will bounce. There will be a bounced check fee.

Electronic transactions can "bounce" just like a bad check. When the $700 transaction tries to clear your bank account and your account is short on funds, it will "bounce". When it does, your entire bank balance will become negative and your bank will expect you to add enough funds to cover the rest of that transaction. Any transaction that comes in after that $700 transaction will also "bounce" and you will be charged over draft fees for every transaction that bounces. The $500 transaction will not clear since the $700 placed your bank account into a negative balance.

Cancel the $500 transaction if you can and then contact the car company and the bank on Monday to try to reverse the $700 transaction and change it to the $500 on the phone. If you can convince them it was a typing error, they may reverse everything with no penalties. Start with the car company because they need to withdraw the transaction before the bank can fix anything that happened to your account because of the transaction.

First thing Monday, call them.