Why did Russia free an imprisoned US ex-marine after four years?

Does the release of former US serviceman Robert Gilman from a Russian prison come down to the relationship between US President Donald Trump and Russian President Vladimir Putin?
Gilman was pardoned by Russia and sent back to the US after spending four years in Russian custody. Trump says Putin allowed Gilman to return home on humanitarian grounds, without any exchange of captive Russians.
DW's David Levitz talks to Craig Unger, a journalist who has followed Trump's relations with Putin closely. He's the author of two books about Trump, including 'American Kompromat' and 'House of Trump, House of Putin' and a proponent of the theory that Trump is a Russian asset. Unger says Putin may be using Gilman's release as a favor to call on later.

00:00 Russia releases US ex-marine Robert Gilman
00:25 Gilman's family says he was abused
01:04 What does Gilman's release say about Trump and Putin's relationship?
01:58 Author Craig Unger describes the "favor bank"
02:30 Unger says Russians have helped Trump time and again
03:00 Why Trump needs a win
04:02 Dissonance in White House messaging
04:45 Payback could come later
05:09 Why does Trump often praise Putin?
05:45 Unger says Russians bought apartments in Trump Tower
08:00 No direct evidence of compromising material
08:15 Unger says Russian spent a billion dollars on real estate
09:05 Why isn't Trump pushing Putin to end war?
10:19 What favor could Putin request from Trump?
11:25 What does Gilman's release say about US-Russia relations?