so far the discussion has mostly been about whether one should or may do this.
Another point is whether one CAN do this.
The story is reported as though it is a simple matter, like transferring some funds.
That being said, I actually do know of a story where someone offered to give years of his life to a sick godol. I will not tell the story here, only point out that it was a private matter, only told to his family because of something that occurred afterwards.
the story was only made public at his l’vaya many years later and only because of the follow up.