How can I know item stock, if I buy the rice per sack and sell it per kilo?

You need to create 2 items.

1. Create 'Sack of rice' (for example 1 sack = 10 kilo of rice).

Select 'Sold by each'.

2. Create a composite item 'Rice, kg'

- Select 'Sold by weight'
- Turn on 'Composite item' in 'Inventory' card.
- Add 'Sack of rice' item to components table and enter quantity 0,1 (because 1 kilo = 0,1 of sack)

Now you can sell both items: 'Sack of rice (10kg)' and 'Rice, kg'.
And when you sell 'Rice, kg' you need to enter quantity manually in kilograms.

Item stock of 'Sack of rice (10kg)' will be counted automatically in sacks (including sold 'Rice, kg').