But : the negotiation would have run very very different if they would have said :we buy it, but will never use or publish it!

He might have still sold (for a higher price) or rejected the offer.

You can't change the "purpose" of a contract after it was made.

In german, we have a law, yes a law, basically saying : in treu und glauben (in good faith / in trust and believe) it basically prevents contractors to write contracts which fuck over signers.

It is super rarely used for misuse of contract law / information of the signer, but sometimes it is. This is probably one of those cases.

In good faith, that Spotify will publish his work, he signed the contract.