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Solution: Use Debit Note with Invoice Correction type as shown.
1. As Invoice Received created of 4,000 AED for 2 items with different quantity.
Refund of amount of 500 AED with total payable amount 3,500 AED (invoice received is not paid yet) is required.
See also 4.2.3.1. Invoices Received (Receipt From Supplier)
2. Create Debit Note using generate option, select Invoice Correction type:
See also 4.2.6.2. Debit Notes (Goods Return and Invoice Correction)
3. At the Main tab, enable Amount Only option, and make sure that Basis is filled with the correct Invoice Received and Transaction type (Invoice Correction):
See also 4.2.6.2.1. Creating a Debit Note. The Main tab
4. At Inventory tab, proportionally(!) allocate the amount that should be deducted from item cost:
See also 4.2.6.2.2. Creating a Debit Note. The Inventory tab
5. At Settlements tab, press Fill by Balance and make sure that Basis Invoice Received was picked to the list as the amount 500 AED will be reduced from payable balance of the chosen Invoice Received:
See also 4.2.6.2.4. Creating a Debit Note. The Settlements tab
6. Debit Note creates the following transaction:
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