Daniel did not return on Monday. On Tuesday, a new customer, Luis visited the gallery. Luis offered to purchase this painting at the price of $5,100.$ Luis said that he would return the next day to pay $5,100$ and collect the painting.Alex told Luis that the refund period is $28$ days. Luis agreed with the refund policy. On Wednesday, Luis did come to the gallery to pay the $5,100$ and collect the painting.Required:Explain whether there is a valid contract between Alex and Luis to sell the painting for $5,100.$ In your answer, explain how the agreement between Alex and Luis meets $($or does not meet$)$ the five requirements of a valid contract.