You can take a out a loan from Duke, up to 20% of the money you have made from crimes and received directly from the system when selling items into the game (not to other players). You can also increase this by improving your Credit rating with Duke, which you do by paying back your fees on time. He charges you 2.5% interest per week on your outstanding balance. The maximum loan it is possible to have is $1,500,000,000.
Duke has asked me to make it clear, that not paying your loans back is very a bad idea!
He is also one of the agents featured in missions, and may assign you contracts to complete.
History: Tim was removed as the resident city loan shark on 11/02/15.
Tim Hi I'm Tim, how may I help you? You can take out a loan of up to $100,000 but I will charge you 1% of that loan every day until you pay it back even if you have to go into debt, me and my lads will continue to take cash off of you.
Here you can borrow money from Tim the Loan Shark. The initial limit is $100,000, but this can upgraded using Points. Each upgrade costs 100 Points and increases the maximum loan amount by a further $100,000. This upgrade can be purchased multiple times. Note that an honour bar  is available for this.
The 1% payment is automatically taken directly from your cash in hand every day. The automated payment is purely interest and does not bring down your total loan balance at all. This means your cash may go into a negative total when the interest is removed.