What is the way to mask the “amount” in an invoice?

Probably ned more information, what do you mean mask the amount.

I guess my question would be why

Can’t imagine that being a legal invoice anywhere

Ie prevent overriding the amount

Oh, you want to grey out the amount box on each line in the UI?

use a workflow or script to disable the sublist fields

PNJ , yes I want to grey out the transaction line amount in order not to change by users. I believe we need script

Edit your amount column to have no header - it'll set it to zero width and the users can't modify it.