You owe the money. Contrary to what most people think should happen, early exercise does happen for a variety of good reasons. I have done early exercise once I decided I would hold a position because I could change the tax status I was under by the exercise.
The broker actually had to cover your charge out of their pocket. They paid interest to a bank or to their customers for carrying deposit balances, you paid them. Unfortunately, assignment settles after hours, but before the open. It happens by random assignment from the options clearinghouse.
There is a process the broker-dealer goes through to determine if you are eligible to trade in options and they send you a painful to read document that is the rules for the contracts. I personally do not like this process because they provide you rules without much context. If you lack experience you could read a rule but not recognize its significance. Nonetheless, under U.S. law that is not an excuse or grounds for recovery. Federal securities laws are buyer beware laws. The duty is to disclose the terms and conditions, not to explain them.
What I would do, if I were you, is imagine that you are going camping in an area with absolutely no cell phone service or computer access. Then look at your positions. Consider what could happen under a variety of scenarios, such as the flash crash or the '87 crash. Remember, you can be in a car wreck. What would have happened if that position had been held open a week?