Full retention on Rantanen sounds like a lot, but his real salary is only $6m this year. Half of that is $3m. And pro-rated to about 1/3 (wince there's about 1/3 of the season left) is $1m.
Conversely Hall's salary is $5.25m. Pro-rated to 1/3 is $1.75m.
So they saved themselves $750k in real cash by moving Hall Vs retaining on Rantanen. And also gained ~$1.2m in cap space. While getting paid a 3rd.
They'd probably have gained that 3rd for Hall at the TDL anyways, but with that approach they wouldn't have saved as much real cash. So they come out ahead.
(Note I made an earlier post on this subject and think my math was a bit wrong in that one, even if my point was the same)