My account with Interactive Brokers has EUR as base currency. I have EUR10,000. If I buy 25,000 worth of USD, can I still make a withdrawal from my account in Euro? Can I withdraw in USD? Will I have a negative Euro balance?
You can withdraw funds so long as you have adequate margin. The analogy is if you bought stock with all of your Euros. If you have a margin account you can withdraw a portion of the stocks' value in cash. And yes, this would leave you with negative Euros for which you will pay margin interest.