What is the treatment of sales return under GST law?

If due to any issue, the goods are returned by the customer to the supplier, these can be sent back under cover of a delivery challan

Please clarify whether advances received under the goods and services tax (GST) regime would attract tax and the document that needs to be raised at the time of receipt of advance. 

GST law provides for payment of the tax at the time of receipt of advance towards the supply of goods or services. However, with effect from November 15, 2017, the government has exempted all registered taxpayers, except persons opting to pay GST under composition scheme, from payment of tax on advances received after November 15, 2017, against supply of goods.

It is pertinent to highlight  that exemption from payment of GST