limiting the customer to a value

Hi sd gurus,

i need a clarification for the following scenario, ex: if a customer places an order the value of the order should not be less than 5000/- rs. i should able to raise a sales order only when the value of the order is equal to or more than the amount, if the amount is less than i should get a pop-up, is their any possibility apart from user-exists. points will be rewarded, thanks in adv

mallik