All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, is His slave and Messenger.
The fasting is valid even if the person does not seek the forgiveness of those whom he wronged. However, it is incumbent on him to seek the forgiveness of those whom he wronged before the day comes when Deenaars and Dirhams will be of no avail (i.e. before the Day of Judgment). If his act of injustice involves wrongfully consuming someone’s wealth, it is incumbent on him to return it to the rightful owner. He is not obliged to inform him of the reason (of giving it back); rather, the purpose is to give him back his right. If it involves backbiting or the like, the scholars held different opinions as to whether or not it is obligatory to seek the forgiveness of the wronged person. The chosen opinion by many scholars is that it is not incumbent to seek his forgiveness, and it is sufficient to ask Allah, The Exalted, to forgive him and supplicate Allah in his favor, because informing him of the act of injustice in this case incurs bad consequences.
Allah Knows best.