Is it a sin for the questioner to tell her sister that the apartment's price is 320,0, despite knowing that its price ranges between 320,0 and 350,0?

Truthfulness in buying and selling is a source of blessing, while concealing the truth leads to its eradication. It is permissible to buy from a relative and to haggle without lying or cheating. If you told your sister that the apartment's price is 320,0 as its highest price, while you knew it was worth more, then you have lied and cheated, and you must repent and inform her. Similarly, if you asked a broker about its price and concealed what he said, this is cheating. However, if the sale was made at this price without misrepresentation or claiming it was the highest price, then there is no sin; because there is no lying, cheating, or excessive unfairness, as it falls within the evaluators' assessment.