I suppose you mean he buy in 10kg bags, then sells in 10kg, 7kg, 3kg etc?
This is a simple Bill of Material.
Item1 = Potatoes 10kg - No bill-of-material
Item2 = Potatoes 7kg - bill-of-material "At time of sale" with Item1 Potatoes 10kg - Qty = 0.7 (7/10)
Meaning every time a 7kg is sold it deducts 0.7 of the stock off Item1.
Item3 = Potatoes 3kg - bill-of-material "At time of sale" with Item1 Potatoes 10kg - Qty = 0.3 (3/10)
Meaning every time a 3kg is sold it deducts 0.3 of the stock off Item1.
OR -
Still 3 Items, but Item1 is set up as bag of Potato per kg. Supplier quantity is 1, so when he GRV's, he enters 10 as the quantity, meaning he bought 1 bag. if he buys 50 bags of 10kg, it is 500kg. This is better in my opinion as sometimes the market may only have 7kg bags!
Item2 & Item3 quantities are then set up as whole eg. 7kg from 10kg bag = 7 ....
Makes sense?
ps. Good questions - thanks!