When u received 100 bags entry would be
--Purchases Dr 100 ( This will mk your inventory 100) (Inventory Dr)
--Vat receiaveble Dr (Applicable rate)
---To Party Credit .
When u distribute the same ( You need to take 20 bags out of the stock)
Expense a/c dr ( Expense head can be ; Advertisment, damage as well).
To Purchases cr ( Inventory Cr)
One more way of accounting which is actually a bit fradulent type would be
Inventory a/c dr ( For 80 bags) ( Amt Total bill value for 100 bags ) ( Your per unit cost will go up here)
To supplier ( bill amt)