@Blickers,
Not surprisingly, you are missing my point. "Russia" is not a living, breathing, thinking entity. To the extent it can be said to do anything, what it does is entirely dependent on the people in charge at the time. Putin was not in charge in 1991, or, obviously, when the Crimea was shifted from one USSR pocket to another.
I don't know the details of the original transfer well enough to know whether or not it involved a treaty being signed, but even if it had, such a treaty would have been a farce considering the Ukraine was not actually a sovereign state at the time and the USSR has been cast to the dustbin of history. Regardless of his character, Putin should not be expected to honor, abide by, or insist upon deals made by the USSR. If he was a freedom loving democrat, you wouldn't expect him to. I am also unaware of any actual treaty, between Russia and the Ukraine, which included territorial definition, being signed and ratified in 1991. I feel certain that if one existed Ukraine and its supporters would have been waving it furiously in the air when Russia invaded. If you have evidence of such a treaty, please share.
Clearly, at the time of the Soviet breakup, those in charge of Russia either didn't believe they had a territorial claim on the Crimea or they saw no benefit in pressing it. It certainly doesn't defy credulity to imagine that there were plenty of Russians, at the time, (including perhaps a younger Putin) who didn't agree with the decision, but had no power to alter it.
In the absence of a treaty that could be taken seriously, succeeding Russian governments were not bound by a decision not to press a territorial claim in 1991. Again, I also feel certain that you are not going to regard the many decisions and actions of President Trump binding on future Democrat presidents. Should he manage to negotiate a deal with North Korea, unless it involves a treaty ratified by the Senate, the president who follows him (Democrat or Republican) will be perfectly within his or her rights to render it null and void. It's what happens when a president makes foreign policy moves that are not broadly supported by the American people (as determined by their elected representatives) and which is why Trump could dump the Iran deal as well as the Paris Climate Change Agreement.
I guess it was worth a shot by Obama to enter into these two deals without Congress giving them their stamp of approval, but since he didn't take the route that would have made them more entrenched, Trump was able to sweep them away with the same phone and pen Obama used to enter into them. So many people were counting on Hillary to keep these deals in place...including the Iranians.