Is it permissible for me to purchase an item if the seller has pledged to refund the money to the first buyer in the event that I pay more, knowing that the first buyer has expressed disinterest in the item?

It is impermissible to sell over another's sale or to buy over another's purchase, due to the Prophet's prohibition thereof in his saying: "None of you should sell over the sale of another, until he has bought or given up." The form of selling over another's sale is when a person offers a similar commodity at a lower, equal, or higher price after the two parties have agreed. The form of buying over another's purchase is when a person offers to buy the commodity at a higher price after the two parties have agreed. This prohibition is due to the enmity and hatred it causes.