Is it permissible for me to sell an item to a second buyer at a higher price, after I have agreed to sell it to a first buyer at a lower price, especially if the presence of the first buyer is uncertain? And is this considered a sale over a sale?

The prohibition against selling over another's sale or bidding over another's bid applies when there is mutual agreement or reliance between the parties regarding the price and the merchandise. However, before such reliance, during the negotiation phase, the prohibition does not apply. Therefore, if what the questioner did was merely a negotiation, there is no harm in selling the merchandise to another person. However, if there was mutual agreement on the price and reliance on the sale, then this falls under the prohibited bidding, and it is not permissible to sell it to another person, even if they offer more. It is permissible for the questioner, upon mutual agreement and reliance with the buyer, to stipipulate a specific period for the completion of the sale, otherwise, she may sell it to someone else.