“It is hard to ascertain that the storeowner has no loss whatsoever when you exchange a sefer,” replied Rabbi Dayan. “First, it’s possible that the original sefer is in lower demand and the storeowner was happy to see it sold off the shelf. Second, the profit margin is not identical on all sefarim; he may have made more on the original sefer. Third, you might have decided to purchase the other sefer in the future and he would have made additional profit. Moreover, some maintain that geneivas da’as applies to misrepresentation even if there is no monetary ramification.” (See Hilchos Mishpat 228:6)
“So what should I do?” asked Benzion.
“You should be upfront with the storeowner and explain what happened,” replied Rabbi Dayan. “He will likely allow an exchange. In rare cases, he might even be willing to grant a refund.”