To rob someone you must have a bank account. Don't have one? Make one.
You must also have a Money Bag but you can obtain this from the shop.
Once you have the Money Bag you can rob someone for how much ever money you desire - as long as you have that amount in your bank account. If you get caught - the money amount you tried to rob will be deducted from your balance. After every robbery you will lose the Money Bag and you must buy it again to do another robbery.