There is a simple and clear statement in Russian law about exactly this situation. Article 1102 of civil code "unlawful enrichment".
It they insist or somehow twist your arms, it is article 16 of consumer code "invalidity of contract terms that infringe the rights of the consumer". I am sorry don't you have some kind of parallel law in your country?
It, very very simply, states "you absolutely can choose what to buy and what not to buy from seller and if somehow you can not, seller pays you difference".