You cannot rollback the purchase, there is opportunity cost and other factors that make this impossible, but a new arrangement can be made if both parties agree to it.
A refund is one way to attempt to satisfy your obligation of the purchase. If both parties to the sale agree that it is satisfactory then it is sufficient. However, one party cannot unilaterally make that decision precisely because the transaction is completed and cannot be undone because the buyer can't be guaranteed to be made whole again by this action.
For example, a buyer choosing between two products both on sale for today only purchases product X for $10 over product Y for $15. If product X turns out to be unable to do the things agreed to in the purchasing contract, and that flaw is discovered a few days later when product Y's price has gone up to $1000 then refunding the $10 and leaving the purchaser without any product has COST money due to no fault of the purchaser and the purchaser probably would not agree to have a refund as a suitable next step, instead preferring to have the original obligation satisfied as promised and paid for.
A more real-world scenario happened to me when I was purchasing an oven from Sears. I bought a floor model for last year's model of oven on sale (but not "As-Is" or "No Warranty"). I paid around $1200 for the oven, which had all the features I wanted and was the price I wanted to pay. I received the oven, installed it, verified it worked and subsequently donated my old oven. A few days later, the new oven's electronic display started to fail. The item was still covered by the manufacturer's 90-day sale warranty so I called the manufacturer service out and they attempted to repair it but were unable to do so. They indicated that they had stopped making parts for this oven and it could not be repaired. They offered to refund the purchase or provide me with a new oven whose price did not exceed what I paid for the oven I did purchase. This would leave me with an oven of less value than I purchased, or no oven at all -- both worse off than before I engaged in the purchase. The sale was complete, I held up my end by providing all the cash requested for them to provide a working oven with the features I had selected. We had come to an agreement and because the product they delivered had a defect they were attempting to substitute some alternative product instead of the one I purchased. I did not accept this proposal, even though they insisted it was the only two options. I offered to contact my state's attorney general regarding this situation. They were suddenly able to send a more competent repairman who reported that this was a common problem with this model, a known-defect, and there was a standard kit to fix it. It was fixed a few days later and has continued to work to this day.