Ideally, would not 'update' stock table. Stock on hand would be calculated when needed. Review
http://allenbrowne.com/AppInventory.html
I still don't understand the need for tblPONum.
Cannot have related primary and foreign key fields both be autonumber. tblPONum and tblPurchases are not properly linked, nor are tblPurchases and tblPurchaseDetails.
Use form/subform arrangements for entering purchase details and sales details.
http://office.microsoft.com/en-us/ac...010098674.aspx
Main form bound to tblPurchases, subform bound to tblPurchaseDetails with combobox to select products
Main form bound to tblSales, subform bound to tblSalesDetails with combobox to select products