Casa Grande Ruins National Monument Boundary Modification Act of 2020 This bill authorizes the Department of the Interior to acquire specified private or state lands or interests in land from willing sellers in order to expand the boundaries of the Casa Grande Ruins National Monument. As consideration for the acquisition of the lands, unless such lands are donated, Interior must (1) pay fair market value; or (2) convey to the land owners federal lands, interest in federal land, or any other federal asset of equal value located in Arizona. Administrative jurisdiction over a parcel of approximately 3.8 acres of land is transferred from the Bureau of Land Management (BLM) to the National Park Service (NPS), a parcel of approximately 3.7 acres of land is transferred from the BLM to the Bureau of Indian Affairs (BIA), a parcel of approximately 7.41 acres of land is transferred from the BIA to the NPS, and a parcel of approximately 3.5 acres of land is transferred from the NPS to the BIA. Interior may enter into an agreement with Arizona to provide for cooperative management of approximately 200 acres of specified state trust lands.