It is possible to earn more than 9999999. This is however the limit of the display so you will not see the full amount you have. Try fooling your friends into thinking you have infinite cash by having more money over 9999999 than the cost of an item. When you buy the item , the indicator will spin and return to 9999999 so you seemingly haven't lost any money. Mordecai is best for earning cash - low damage bloodwing. Full drop extra items and bloodwing attacks multiple targets + good scavenger mod works brilliantly
While it is possible to earn that much money via Mordecai it's much easier just to do so via item farming. Here's a tutorial:
1 >> Open a Single Player Game in New Haven
2 >> Open all 5 chests in town and collect all the items.
3 >> Sell the items you don't want in the store and check the store for items
4 >> Save & Quit
5 >> Do steps 1-4 over and over until you've found all you want
Here's where the chests are: http://borderlands.wikia.com/wiki/New_Haven:_Weapon_Crate_Locations