Han winning the Falcon and what Lando was supposed to be upset about are two separate things. As someone said it's covered in the Crispin books.
Here's how it's described in the appropriate wiki entry:
Solo's heaviest involvement with the fledgling Rebel Alliance occurred when he and other smugglers, including Lando Calrissian and Jarik Solo aided Bria Tharen's Red Hand Squadron in raiding the Ylesian colonies and putting them out of business—a plan endorsed, aided, and supported by Jabba, a business rival of Ylesia. In the subsequent battle, the Alliance forces were victorious, but several smugglers were killed. Han and his smuggler friends were double-crossed by Tharen who seized all the spice and treasures that were to be the smugglers' reward for the Alliance.
Soon afterward, he met with Tharen again during a Hutt-sponsored treasure hunt for the Yavin Vassilika. Once again, Tharen took the prize for the Rebellion, while Solo and his smuggling associates were left with nothing. Solo and Tharen left on poor terms on both occasions, since Solo refused to give up his independent ways and join Tharen in the Alliance. However, most of his fellow smugglers assumed that he had been working with Tharen from the beginning. Even one of his closest friends, Lando Calrissian, said that he never wanted to see him again.
It loses a lot in that bare bones description of the events. The book is actually very good from what I remember. I'd recommend them to anyone. What it also leaves out is that Tharen and Solo were both in love with each other once upon a time. As I said you have to actually read the books to get the full context.