Whenever you complete a side quest that doesn't involve a cut scene, go to the person where you have to give them the item, however draw your weapon first before you talk to them and as soon as you automatically put away your weapon, then just press A as quick as possible.

Don't stop because if you do then you will have to find a different side quest to redo this cheat.

You won't get much cash however it will build up.

Even when you're level maxed out, you can still press the button for continuous cash.