"... negotiations have not gone through yet. --- Are you still in contract? If you are, you may want to check with the broker of your realtor to see if he can assign someone else to work with you to complete this transaction. Although your realtor has quit, your agreement is still with that agency.
",,, our contract expired before I found the property" --- if you're referring to buyer agency, review that contract to determine what are the obligations if any, once you enter into a purchase contract that while you were in that contract time period. The reason being, if that realtor wrote up the contract for you and it was accepted, he can claim to be the procuring cause. Whether you can continue using him or not, he may claim the commission on that particular sale.
If the broker assigns someone else to help you, that may be the better scenario at this time.
It would most definitely help to put a cancellation in writing --- and make sure that realtor's broker receives it.
But as far as who gets the commission --- that may be something you can negotiate with the broker if another agency picks up where the old one left off.