How to add an advance payment field on Order that also updates the extended amount ?

Hi, 

Our customer has a requirement where he wants to have an advance payment option either in a separate field or on the Pricing fields set below:

For example if I have a total extended amount of $5000, the customer paid $1000 in advance so while creating the order, I should be able to add $1000 as advance and the remaining amount will be $4000, this should also reflect in invoice. I know CRM is not designed this way as this is the function of the ERP or sales system but this is customer's die hard requirement. 

Please advise. 

Thanks,