Down Payment Problem....

Hello Experts,

when doing an A/P Invoice(item type or Service type), i want to alert the user if the PO has an A/P Reserve Invoice reference in to it. so that the user is aware that the PO/GRPO that the user making an A/P invoice has a down payment. also in the in the "Down payment to draw" window there is a huge list of down payment done, how i will identify the down payments base on specific PO? (sometime down payment in one PO is more than 1)

Please guide how will i do this.

Thanks....

FIDEL