Heat the oil over medium-high heat until hot and cook the beef and garlic, stirring with a wooden spoon until the beef is no longer pink. Remove the beef from the pan with a slotted spoon and pour off the fat. Return the beef to the pan and add the ketchup, tomato paste, mustard, brown sugar, and chili powder. Stir well, add 3/4 cup water, and cook over medium-low heat for 5 to 6 minutes or until everything is combined and slightly thickened. Add the salt and season to taste with pepper. Spoon the warm Sloppy Joe filling over the hamburger buns.