What is the ruling on marrying a woman who was previously engaged to another man, and you helped her break off that engagement, knowing the hadith: "None of you should propose to a woman to whom his brother has already proposed," after the act has already occurred?

A Muslim proposing to a woman already engaged to another Muslim is forbidden, due to the Prophet's (peace be upon him) saying: "A Muslim is the brother of a Muslim. It is not lawful for him to outbid him in a sale, nor to propose to a woman already engaged to him, until he gives up." Sin is incurred by whoever is aware of this prohibition, and the marriage is valid, though sinful, according to the majority of scholars.