Why would you have "used" a Realtor you had no intention of working with? Do you realize that we are not
paid a salary, and, therefore, our time is precious? If you use one Realtor to do the legwork, and write the offer with another Realtor, I'm sure you can see how this is patently unfair. Now, to answer your question, if you do not want to work with a particular Realtor, you are not required to. I, for one, would not want to work with a client who didn't have faith in me! However, the first Realtor may have claim for "procuring cause", and the second Realtor may have to pay part or all of the commission to the first if an arbitrator determines that to be the case.
Please, buyers and sellers, be honest with agents! I expect Realtors to be honest with clients, and would request the same courtesy. If you want to work with the second Realtor, please tell him or her this situation, so that they will know the whole story and can make an informed decision on how to proceed.