By Charlotte Greenfield and Jonathan Landay KABUL/WASHINGTON (Reuters) -U.S. and Taliban officials have exchanged proposals for the release of billions of dollars from Afghan central bank reserves held abroad into a trust fund, three sources familiar with the talks said, offering a hint of progress in efforts to ease Afghanistan’s economic crisis. Significant differences between the sides remain, however, according to two of the sources, including the Taliban’s refusal to replace the bank’s top political appointees, one of whom is under U.S. sanctions as are several of the movement’s leaders. …