Well, there is nothing wrong with trying to negotiate. However, if it is true that you have an agreement, and if it seems your partner has fairly respected his

Well, there is nothing wrong with trying to negotiate. However, if it is true that you have an agreement, and if it seems your partner has fairly respected his part of the agreement, I don't think you would have any legal case. As an owner of the business, he took on all the risk, paid for all the costs. You agreed to be paid a royalty.