The possibility that the seller can get a better price is not your problem. That would be the seller's duty to mitigate damages -- it must accept the best offer available. If they have asserted an opinion in writing that they believe that they can get a better price, and that is the reason why they are rejecting your assignment, then they are just providing better evidence for you.
As far as your "email purchase and sale agreement" goes, if there is a bona fide agreement between you and the buyer in which you agree to deliver a specified quanity by a specified date and for a specified price, then you can assign that agreement to the seller. If the seller refuses to accept, then it will have failed to mitigate damages.
Note1: you must notify the seller that you do not intend to pay the $20,000, although you don't have to state the reasons why, and that you are offering the $12,000 assignment as a substitute contract. The seller needs to be on notice that you don't intend to perform -- otherwise, it can claim that it didn't need to mitigate damages, because no damages had arisen at the time of your offer to assign the contract.
Note2: If you have a bona fide contract with the buyer, then I don't know why you would have any qualms about revealing the buyer's identity to the seller. It just demonstrates that there is no "funny business" going on.
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