Downpayment Processing at Customer Level

Hi,

I have a requirement to implement Down Payment Processing at Customer level and not at sales order level. In this, client wants to create a down payment request & clearing at customer level (may be by raising a separate sales order only for down payment) and then this cleared down payment should be adjusted to any billing for this customer that we do in future based on FIFO method.

Please guide me how to proceed for the same.

Regards,

Naveen Aggarwal