Unfortunately, no. Once a bonus letter, offer letter, employment agreement, etc. says something like "are determined at the sole discretion of the Company and no bonus payment is guaranteed," the company has FULL discretion to offer you essentially whatever it likes--as much as you may like to hear this, a "sole discretion of the Company" bonus is essentially at the company's whim, and the offer letter is meaningless in terms of mandating or guaranteeing a bonus. That language preserves the company's unfettered right to determine your bonus to be anything it wants.Being pro rated for months worked, by the way, is actually quite common and fair--it's not something I would recommend a person complain about.Going forward, though, next time to negotiate something like this, try to get rid of "sole discretion" type language.