Why my purchase partially allocated to Cost of Sales account in the journal??

Hi Shunya 

Updating the price on a bill after the fact will cause the cost of sales value to show on the Bill when it typically would not show. This caused by the fact that the system re-calculates the average cost for that item and then redraws that cost of sales amount.

Based on my testing, it does look like it works out the appropriate value with the calculation sold being:

Purchase Cost of Sale amount = (Purchase cost per unit - Average Cost) x Quantity Sold.

Although, long term for that item, updating the bill after selling those items is not something that I would recommend for users. While it is likely and does appear to calculate the desired amount when that item is sold, it may have lasting long-term impacts, especially if the item is being sold in chunks. The recommendation if you have received items without a supplier bill then Help Article: Recording items received without a supplier bill would be the recommendation.