Do you have access to tanning supplies where you are. Not orange bottle, krowtan etc, but products used by commercial dressers?
If you can get a good answer on what has actually been done to it, that would be a huge benefit because it will eliminate a lot of variables.
Basically the "brain tanning" process creates aldehydes as the brains break down, which will help preserve a skin. The problem you are going to run into with something that is very fatty is, if it was not degreased well, the natural fats left in the skin oxidize and turn into fatty acids. That is where your yellow color comes from, and it also is what is the biggest culprit in what people refer to as acid rot or hide rot. Polar bears are one of the worst fats there are for going rancid in a hurry, that's why you see so many with orange stained under fur.
If you cannot get a good answer on what was already done to it, try wetting a small spot to see if it will relax at all. You'll want to add about 1/4 lb. of salt per gallon of solution (you'll need to do the conversion) and preferably a good commercial relaxer. Check with taxidermy suppliers up there and see what your options are, you should be able to buy small quantities.
If it is going to relax, then you can proceed to soak and tan it properly, but the stain on the fur is going to be difficult to remove without bleaching at this point. If you have never handled anything like a bear before, you might want to talk to a commercial dresser up there and see if they will do it for you. We do a lot of salvage work here on skins that were attempted at home with any of the "quick and easy" methods and can usually get decent results.
I don't login here a lot, but will be glad to help talk you through it if you want.
