First of all, I would not cancel it so fast.
What I would consider in this case is coming up with another option for your client.
Talk with your client by saying, okay hey listen, we agreed on this and that for this price, you are asking now to do this and that but it is an addition to what we agreed on in this order so I will have to request a little extra to do so.
If you want I can set up yet another payment so I can finish the additional work that you are requesting now, Would that be fine with you? Otherwise, we will have to do this in the next order.
And with this "We will have to do this in the next order" You are saying, okay I can do it for you but it requires additional cost, and if you don't want to do this now you can always request another order later. And with that, you are giving your client two different options.
He can decide to move on with it
He can say, nah that is for next time then.
The problem was solved, and you did not have to cancel anything.
It is all in the way how you bring things to your customer, don't react aggressively, just come up with solutions that will fit both of you.
Succes!