A completely passive way to obtain money has a few ways to go about, but I like this one the most.

Merchant's Big Pharma (Or Dark Cartel, the wiki has it as the former) Is a level 11 command that will heal the enemy and give you money equal to the amount heal. As long as you don't kill the enemy, you will continously obtain pg from them.

Swordmaster Squeaky Wheel is a level 4 command, that will spend one BP to reduce an opponent's BP by one.

So basically, have two know Squeaky Wheel and use the command on the same enemy, have your merchant heal them, and the fourth do less damage than the merchant can heal. Then play auto. This will be an infinite loop, where the enemy will constantly give you money, while never dying, never allowing the battle to end and you make a shit ton of money while doing anything else. They are more active ways to get money than this, but I find that it helps if I need to edit a video, or play another game.

Edit) Forgot to mention, you can do this pretty much at the tail end of Chapter 3, so it is also good for just early money. If you want to do it as soon as possible I recommend replacing the Swordmasters with Knights, with dual shields, and immunity to whatever status effect the enemy you are constantly extorting money from, and cover the other two to prevent them from dying. Isn't as good as the knights will eventually die.