wants a R15000 deposit on a second hand car for what???

trying to buy a second hand Nissan in Durban they app**** to banks absa agreed to loan as did wesbank but wesbank wants a R6000 deposit absa wants R15000 when I ask the dealership whats this for? is this a deposit to be taken off the cost of the car they say no...... so why must I pay R6000 to wesbank or R15000 to absa and since I a am with absa why do they require me to pay R15000 when I already have assets with them????? Explain