If there is a disagreement between guests and guides on fees without any sudden changes on the appointed day, generally you do not need to pay extra. The fact that the tour is being done on that day means that the offer and reservation of the trip are done after both guides and guests agree on it. If you have any problems with the price, discuss it in advance.
However, if you decide to change the place you want to visit on that day and you consult with your guide and they accept, additional fee may be incurred. In this case, just directly exchange money on the day under mutual agreement.