What is the sin incurred by one who lies in such cases, and is the suitor sinful if he declines to propose to the girl based on hearing negative talk from one party without verifying its authenticity?

There is no blame on the suitor for withdrawing from the engagement due to what he heard about his fiancée. However, it is better for him to verify, based on the Almighty's saying: "O you who have believed, if there comes to you a disobedient one with information, investigate, lest you harm a people out of ignorance and become, over what you have done, regretful."

As for whoever lied about the girl and spoke falsehood and slander against her, his sin is great, due to the Almighty's saying: "And those who harm believing men and believing women for [something] other than what they have earned have certainly borne slander and a manifest sin," and the Prophet's (peace and blessings be upon him) saying: "Every Muslim's blood, wealth, and honor are sacred to another Muslim." Describing a person with something that is not in them, especially concerning their honor, is among the slander and falsehood that necessitates repentance and seeking forgiveness.