My client asks for work that was not described in the original task description for the fixed price we agreed upon.
I already explained it to him why can't do it for the same fixed price (because I gave the price only for the things that we originally agreed about) but he's still pushing the envelope. Whats' the best way to amicably take care of that?
re: "client asks for work that was not described in the original task description"
Clients may not do that.
If the client wants to ask for things outside of the original task description, then the client needs to:
a) close the current fixed-price contract and create an hourly contract
b) create a new milestone for you to do the additional work
re: "...he's still pushing the envelope. Whats' the best way to amicably take care of that?"
I think this is something that is faced by EVERY freelancer who works on fixed-price contracts.
It takes wisdom, thought and experience to handle the situation well.
But you are on the right track, because you know that you MUST handle this.
You can't let clients do this sort of thing, because if you let them do it even a little bit, it can completely ruin your business as a freelancer.
My advice: Be polite. Be professional. But remain firm.
And remember: This is not just your rule. This is Upwork's rule. Upwork has a STRICT RULE prohibiting clients from asking for free work. You do not need to tell the client that he is asking for free work. But that is what a client is doing any time the client asks a freelancer to do anything outside of the original milesone agreement: The client is asking the freelancer to work for free.
You could tell him:
I completely agree that this is a good idea. I want to work on this for you. It will make the project better. Upwork has pretty strict rules about this. So because this step is outside the scope of the original description, I can not do it as part of the current milestone. But if you set up a new milestone for $50 I can do it. Or you could close the current contract and set up an hourly contract. Please use whichever method works best for you (new milestone or hourly contract), and then I'll be able to get to work on this immediately."