How does one return rights to a partner who betrayed me, and whom I then betrayed in kind, given that the amount is unknown? And is it sufficient to seek forgiveness in a general manner to be absolved of these rights?

Betrayal is not a characteristic of believers; rather, it is a characteristic of hypocrites. It is a blameworthy trait, and its perpetrator is not beloved by Allah Almighty. The author of "Az-Zawajir" considered it one of the grave sins. If there has been a betrayal, it is incumbent upon the one who committed it to offer sincere repentance and to return the money taken unjustly. The obligation is not fulfilled until it is returned. It is permissible to return the money without informing the other party. If the amount of money is unknown, then one must strive to estimate it and pay what is most likely to have been taken.