If your written agreement with them provides for this, then you may have to pay. But if there is no written agreement on the point, then you probably should not have to pay it. But lawyers here on AVVO are just speculating about the answer, because none of us have all of the necessary information to give you a definitive answer. I do not think that there is anything in the Workers' Comp Act that would require you to pay them after the fact. It is likely going to be determined by whatever is in your contract with them.
This answer is intended as general information and not as specific legal advice. If you want to have a free consultation with me, please contact me through AVVO.