It is mentioned in a Hadith that you should not propose to a girl when your fellow Muslim brother has already proposed to her until he gets married or gives up this proposal.
In other words, when a person has sent a proposal to a particular family and there is a likelihood of their replying in the affirmative, another person should not send a proposal to that same family. However, if they reject this first person, or he himself changes his mind, or they are not too happy with him and are still hesitant in giving a reply, it will be permissible for another person to send a proposal for the same girl.
The same rule applies to the transactions of buying and selling. That is, if a person is busy buying or selling something, then as long as they do not separate or abandon the transaction, another person should not enter into their transaction and should not offer a price above or below that which has been already offered wher there is an indication that they are about to come to an agreement.
Understand this well, and know that a non muslim is also included in this rule (with regards transactions of buying and selling)
Article from ‘The Shari’ah’ newspaper