Check with your Accountant for the standards that apply to the country you are in. 

Example: You order 10 units at $10 each and receive 11

You have $110 in current assets and a liability for $100.

You can either

1. record the difference as other income (* may have tax implications)
2. re-value your inventory to $100

In both cases, everything is the same until you process the Vendor Bill:

• Purchase Order for 10 units at $10
• Warehouse Receipt for 11 units

Note: at this point you have 11 units at $10 each in Inventory - a valuation of $110 - it is important to record every material asset you receive

Vendor Bill:

• debits the interim account for $110 with a line showing 11 units at $10 each 
• credits the other income account with a second line showing 1 unit of a -$10 discount

After validating the Bill, your interim stock account is balanced. You have $110 worth of inventory, a $100 liability and $10 of other income.

This is where you stop if you elect option (1).

If you elect option (2), you use the Add Manual Valuation feature from your Inventory Valuation Report (a plus icon next to the Product):

Be sure the Added Value and Counterpart Account are the same as the discount line on the Vendor Bill.

After this you have $100 worth of inventory, and a $100 liability.

Note: if the Vendor Bill was already incorrectly processed for 10 units at $10 and your stock interim account is unbalanced, create a second Bill as outlined above debiting the stock interim account for the value of the extra products received with a second line that credits your other income account - the bill should net to $0 because of the discount line you add.  THEN complete the final step of revaluing your Inventory.