Does the prohibition of selling deferred for deferred include a person returning an item he bought, then re-buying it to benefit from a discount that appeared on the item on the website, knowing that he bought the second item before the completion of the first sale?

If the store allows returns, there is no harm for the buyer in returning an item and purchasing the same item at a reduced price. There is no connection between the two contracts. He may buy the discounted item first, then return the original one, provided he does not claim a non-existent defect or engage in falsehood. This does not fall under the prohibited "sale of debt for debt" (bay' al-kali' bi al-kali'), which is selling debt for debt or deferred payment for deferred payment. The first sale is complete, and the second purchase is an independent contract.