Opal is server side currency, you cannot change that.
If you can buy it with real money or get it from special currency in game as an alternative, it's usually server side, They learned that real fast after AC Origins where everything was for gold.
I forgot to say : i tried what you suggest, nothing works neither
What they learned is people want it but doesn't wanna give them more money for it lol so now its the only way unless you cheat it.
Here's how I got it to work for me: First, make sure you are using the latest trainer. Second, play the game in window mode instead of full screen so you can easily alt+tab between the game and the trainer. After you load the game, activate the trainer. Now open you inventory and click the item you want to edit. Press alt+tab to swtich to the trainer and edit the value. * Now, click somewhere in the trainer! * This will set the value you just entered. Now alt+tab back to the game, and don't do anything. Just press the back button to exit out of the inventory menu. Now if you go back to the inventory you should see that item value has changed.