ERPNext has an option for products that are need to be returned to the supplier. This may be on account of a number of reasons like defects in goods, quality not matching, the buyer not needing the stock, etc.
You can create a Purchase Return by simply making a Purchase Receipt with negative quantity.
First open the original Purchase Receipt, against which supplier delivered the items.
Then click on "Make Purchase Return", it will open a new Purchase Receipt with "Is Return" checked, items and taxes with negative amount.
On submission of Return Purchase Return, system will decrease item qty from the mentioned warehouse. To maintain correct stock valuation, stock balance will also go up according to the original purchase rate of the returned items.
If Perpetual Inventory enabled, system will also post accounting entry against warehouse account to sync warehouse account balance with stock balance as per Stock Ledger.