Commission earned on final check

[USBCDIVA]

What is the name of your state? CO

I have recently resigned at my place of employment and my last day is July 3rd. I gave 5 weeks notice to my employer as I know my position is difficult to fill. I am a salaried employee that also receives a commission on the gross sales for one particular event account that I manage. The event will be complete on July 1st allowing me to know the gross sales before my departure. The company pays royalties to the client on this event, so we are not waiting for payment from the client to consider this event over.

When the original terms of the commison were set up, the payment of the commission was to be paid in three equal installments over a period of three months beginning in August. Since now giving my resignation, I believe that the employer is legally bound to pay the full commission on my final paycheck (which will be July 15th), much like all other wages and vacation I've earned. My employer is attempting to withhold payment of the commisson stating that while my situation has changed, his has not and therefore he intends to stick to the 3 month payment schedule, even though I will no longer be employed there.

In years past, this commission has been difficult enough to collect (two out of the three months last year he "forgot" to give the info to payroll) and they have "delayed" my payments. Naturally, I am skeptical that I will receive this large sum of money if I am not around the office to collect. MY QUESTION IS: Is my employer required to pay my earned commisson on my final paycheck or are they allowed to follow a post-employment payment plan as suggested?

Your comments are greatly appreciated and I thank you for your time.
What is the name of your state? CO