grammaticality - ... is done in agreement with xxx?

Background:

I'm writing a professional (technical) report in which I want to express the following in one simple sentence: The whole report is written based on a certain assumption, except one part that's based on a different assumption. (Let's say a battery has 10 Volts in parts of the report, and 12 Volts in another part). This means that one part of the report is "wrong". I want to express that the way I'm doing this has been "approved" by our client. The part in parentheses is just to illustrate the context.

(In most of the report 10V has been used. However, the voltage is actually 12V.
12V has been used in chapter 2 only.)

The usage of a different voltage in chapter xxx only, is done in
agreement with client name.

However, the last part of that sentence doesn't seem good to me. I think it's grammatically correct, but not something a native English speaking person would write. Can I say / write: "... is done in agreement with"?

Answer

What you currently have boils down to The usage is done. Usages generally aren't done, but they can be agreed upon.

The usage of a different voltage in chapter x only has been agreed with the client.

I wouldn't use a comma to separate "The usage" from "has been agreed". You could also lose only as that can be inferred from the context.

The distinction between usage and use might be commented upon.

usage
The way in which a word or phrase is normally and correctly used
Habitual or customary practice, especially as creating a right, obligation, or standard

use
The action of using something or the state of being used for a purpose

[Both ODO]

Usage is about the way something is used; use merely means that it has been used (this is evident in the title "English Language & Usage"). I would write the sentence as

The use of a different voltage in chapter x has been agreed with the client.