His mother gave him up for 2 reasons: To give him anadvantage in life along with financial security  and also to get him away from his abusive father . She said to her husband " to get him away from you"  The Rosebud sled  represented hi lost inocence and  his lost childhood when he was truely happy...in my opinion.

Joseph Cotton is one of my favorite actors of all time.

To answer your question, his mother made a deal with Thatcher for the mine, but also for Thatcher to be Kane's guardian. She did it because Kane's father beat him and we can deduce wasn't a very good father (or husband probably).  The film covers a lot of themes; power corrupts absolute power corrupts absolutely; money never buys you happiness; the need to be loved; etc.  A masterpiece of cinema.

Welles was 25 going on 26. Wow!

Xanadu was based on San Simeon, William Randolph Hearst's lavish estate in northern California.

It's kind of the Star Wars of it's day; the onslaught of special effects in the storytelling was jaw dropping.
There is a scene in Spielberg's Jaws, when they've just caught the first shark and Richard Dreyfuss and Roy Scheider's character's are in the foreground half discussing half debating whether this is 'the' shark and the only way they can be certain is if they did an autopsy. In the background, in the middle of the shot, is the Mayor. It starts out with the Mayor shaking hands, then looking to kinda overhear what they're talking about, going back to shaking hands, moving closer, and the audience sees him getting right up close in the shot. All that created increasing drama. That shot is a 'direct descendent' of one from Citizen Kane; where the parents are discussing Charles going away. Some may find CK a bit boring, but that is only because they've 'seen it all before', only because CK is literally ground-zero of so much of what would become the norm of cinema language and techniques to be used, all debuting here all at once. Orson Welles created a language that no one knew needed creating. So many moments and meaning have been used by directors ever since, heck so many of them relish at the chance of having a movie-within-a-movie, by having a newsreel type footage shown. The mirror repeating a person, a camera zoom in on a sign then going within it and onward, the slow pan from a moment taking place (a singer) towards some people just giving their opinions (the two guys up on the rafters above the stage) all in one shot, on and on...

"It's the well from which we all drink" - William Friedkin.

Some of the things Citizen Kane pioneered were; shots being in deep focus (you can see foreground and background both in focus), the sets had real ceilings (sets at the time in Hollywood did not have ceilings, Welles insisted on building them), as well as its narrative structure and use of sound (such as overlapping dialogue for scenes with multiple characters talking). Those alone combined make it the best film ever made imo.

This was Orson Welles' first movie.

Incredible Oscar-nominated score by the great Bernard Herrmann

What "Rosebud" represented to Charles Foster Kane was his last moment of innocence and having no cares..  He was just having fun, playing in the snow, oblivious to the larger world and all the other people, with their own wants and desires.  He was happy NOT knowing, but his parents sent him off and that was the end of his protected existence.

William Randolph Hearst, who was the basis of the Kane character, was considered the father of "yellow (scandalous) journalism."

The final warehouse scene in Raiders of the Lost Ark is a deliberate visual homage to the ending of Citizen Kane.

See William Randolph Hearst

IMO this is the greatest film ever made. Not only for the incredible performance by Welles, but his direction, which was revolutionary at the time. The camera angles, framing, and shots in this film weren't used in mainstream Hollywood. To get some of the low-angle shots for the scene in the press office, they literally dug into the floor to plant the camera and point up. The non-linear storytelling was also revolutionary. So many elements in modern cinema came from this film.  The AFI ranks it number one, the BFI puts it at number 3.

Welles made some acclaimed movies from Shakespeare: MACBETH, OTHELLO, CHIMES AT MIDNIGHT.

(16:20)  Not many noticed that Kane's wife was taunting him by reading "The Chronicle" newspaper; his competition.  Great reactions!!

the only time you see Kane (IMO) truly carefree and happy, is in the opening scene when he is outside playing on the sled while his parents are signing him away.

This is truly a remarkable work and , I suspect, Hollywood was not ready for it and perhaps the Hurst backlash really kept  Wells from the acclaim he really deserved.  

If you want to see his equal you need to dive into Kurosowa.  Kurosowa was the master of movement inside his films.  The movement of characters, camera, and nature.

His next film was "The Magnificent Ambersons", which is my father's favourite film.  Sadly, the studio hated the version Welles made, and reshot and re-edited it.  I believe that Welles' version is not known to exist in any archive.

Joseph Cotton and Everett Sloan appear in the projection room scene that follows the newsreel sequence.