He bought an apartment from his sister, and told her that the price was such and such, when in fact it could have been sold for more than that

Honesty in buying and selling is one of the means of obtaining blessing therein, as the Prophet (blessings and peace of Allah be upon him) said: “The two parties to a transaction have the option (of cancelling it) until they part. If they are honest and disclose any defects, their transaction will be blessed for them, but if they lie and conceal defects, the blessing of their transaction will be erased.” Narrated by al-Bukhaari (2110) and Muslim (1532).

There is nothing wrong with a person buying from his relative and bargaining with him to get the best price, so long as there is no lying or deceit involved.

With regard to your telling your sister that the price of the apartment was 320,000, if you meant that it was not worth more than that, and that this was the highest price for it, then you have lied to her and deceived her, and what you must do is repent to Allah, may He be exalted, and ask her to forgive you, and you must tell her that it could have been sold for more than that.

Similarly, if she asked you to ask an agent to find out the price of the apartment, then in that case you have lied to her, and what you should have done is tell her what the agent said, because withholding that information is a type of deceit.

But if you meant that the apartment could be sold for that price, without giving her the impression or telling her that this was the highest price, then there is nothing wrong with that, because there was no lying or deceit involved, and there was no selling at too low a price either, because this price is still within the range mentioned by experienced people.

Ibn ‘Aabideen said in his Haashiyah (5/143): His saying “The price that is not within the range mentioned by experienced people” is the correct view, as mentioned in al-Bahr, such as if the item was sold for ten, for example, then some experienced people said that its price should have been five, and others said six or seven. In this case it is too low a price and is too great a loss, because it is not within the price range mentioned by experienced people. In contrast, if some experienced people said that its price should have been eight or nine, and some of them said ten, this is only slightly lower that what the price should have been. End quote.

And Allah knows best.