Not a lawyer, education purposes only, etc., etc., contact a lawyer, etc., etc.
If the terms of payment clearly stated you would be paid X each month for Y months, with X times Y equal to $5k, and they then proceed to pay X times Y equals $5k on the first month and refuse to pay anything for the other (Y - 1) months, then I think that yes, technically they have failed to live up to their obligations under the contract and you have recourse as provided by your agreement and/or the Law.
That said, there are mitigating factors here:
It would seem that this breach is in your favor. If you believe it is not in your favor, you should be prepared to present a compelling case as to how this breach has harmed you. If you can prove damages, then you will be entitled to be made whole.
You have received the payment and, while you complained, you did not indicate you have refused and/or returned the payment. If your actions could be construed as acceptance of the payment in advance, enough reasonable people may believe that you agreed to a modification to the original agreement. That the company doesn't care is just a bargaining tactic; if you care, you need to take steps to protect your interests.
Maybe more. What happens in the event the agreement is breached is determined by the Law, the agreement itself, and finally by what is found to be equitable at trial.