The leasing, sub-leasing and/or sale of a/c between carriers go back a longtime. Example; DAL sold, leased and/or sub-leased a/c to Ozark back in the 60's, 70's and early 80's. OZA leased the 1st four of their MD80's to AA until 1984, SWA leased 727's in their early years from another carrier, AA leased the AirCal 737's to SWA shortly after the AirCal/AA Merger, so leasing/selling a/c to other carriers is not uncommon.
Some carriers use to swap a/c during winter and/or summer too. Those were the good old days as my father would say.