While not a permanent MP3 file you can move between devices freely, these services allow you to "download" the track for offline listening within their respective apps:

: The single is available for purchase on the iTunes Store . Streaming with Offline Access

To legally download "Let Her Go" by Passenger as an MP3, you can purchase the track from official retailers or access it through licensed music libraries. The song, released on July 24, 2012, as the second single from the album All the Little Lights , became a global hit with over four million digital copies sold in the U.S. alone. Official MP3 Download Platforms

: You can buy the digital single or the full album in MP3 format.