Portal summary using two related files

[solaris]

I wonder whether you could help me with the follwing.

I have three database files; purchases, Sales and master.

In the master file I have two portals, the first allows creation of records in the purchases

file using a serial lookup, which can contain multiple records.

However I want to be able to select only the purchase portal data from the master record

into my second portal (SALES) which comprises of two fields supplier & desc., and deduct a sales quantity from the purchase qty providing a balance

remaining.

I have created the related value lists for this but cannot seem to find a way of getting the purchase

quantity to show in the master file.(It is fine and works within the sales file alone).

Your help would be greatly appretiated.

PS: the system will eventually be used on the web.