Contact a licensed real estate agent in your area that is familiar with suburban/farm sales. They can guide you through the selling process while retaining 50% of your mineral interest. Usually the surface rights are sold with the land, and the underground mineral rights can be split 50/50 with the sale. If I understand you, and you own 100% of all mineral rights for the land in question, it can be negotiated to retain them all, but it will probably be a hard sale for the land to be sold without any mineral rights.