We are currently using a rather questionable approach of using charges to handle advance payments to supplier. However I see why payment schedule in Purchase Order might be not a solution for us in Apps 9.
I have set up IP18, and created a payment schedule for a purchase order.
In manual supplier invoice I have matched the payment schedule id of the payment schedule for purchase order.
However the preposting of the purchase order line is not fetched in the posting line of the posting proposal.
Note that we use pre posting on the line and not on the purchase order header. Could you help me troubleshoot?