Trader builds $5 billion position after realizing he wasn't using test program, but brokerage sa

• Harouna Traore, a day trader in France, thought he was using a demonstration version of British brokerage firm Valbury Capital's platform.

• He opened an account with about 20,000 euros, or $23,000, after using the demo during an educational course in Paris.

• Traore racked up a loss of more than $1 million before he realized the trades were real and, instead of quitting, he kept trading for a profit of 10 million euros, or $11.6 million.

He thought he was practicing — but the trades and the millions of dollars of windfall were real.

Harouna Traore, who was taking a class in Paris to become a day trader, used a demonstration version of British brokerage firm Valbury Capital's platform to learn how to trade equity futures last summer, according to the Financial Times. He opened an account at Valbury with about 20,000 euros, or $23,000.

A short time later, he was at home practicing on what he thought was the demo, racking up more than 1 billion euros of orders in U.S. and European stock futures, and losing more than 1 million euros. Then, Traore came to the shocking realization that the trades were live.

"I could only think of my family," Traore, married with two children, told the FT. "I was stressed."

But instead of stopping, he continued to trade, according to the FT's report, building up a $5 billion position in U.S. stock futures and turning his loss into a profit of about 10 million euro, or $11.6 million.

Traore called Valbury a few days later to explain the situation. In response to his skillful turnaround, the firm declared Traore was in breach of his contract and that his holdings were both void and canceled, the FT reported.

Traore has sued Valbury for the $11.6 million, declaring the firm was negligent and had breached his contract.

Valbury, owned by a Indonesia-based financial services company, denies doing anything wrong. The FT reported the firm is expected to argue that Traore was a financial services professional, not a consumer, and that the case should not be heard in French court, where Traore would likely receive much greater protection.

Traore admits he "tried to embellish" his experience on his application to open an account on the site, according to the FT.

This could well be a make or break case for Valbury, as the firm brought in about $10 million of revenue in 2016, according to the most recently filed public documents. Valbury's revenue declined 15 percent year-over-year from 2015, the FT reported.