How can a person who has committed many sins, such as abandoning prayer, theft, fornication, backbiting, usurping people's rights, lying, and not fulfilling vows and oaths, repent with a sincere repentance, especially concerning the restitution of people's rights, and how can these rights and the number of unfulfilled oaths and vows be estimated?

The door to repentance is always open, and no matter how great the sin, the mercy of Allah is vaster.

As for repentance from the rights of Allah, it is done by abandoning sins, regretting them, and resolving not to return to them.

As for the rights of people, injustices must be returned to their owners, even if by strategizing to deliver the right to them without disclosing one's identity if one fears exposure.

As for prayers that were missed, the majority of scholars hold that they must be made up.

As for oaths whose number is unknown, it is preferable to offer expiation for all that was broken by strong presumption, and the Hanbalis hold that a single expiation is sufficient for all of them.

As for vows, they must be fulfilled, and for whatever has not been fulfilled or cannot be fulfilled, an expiation for an oath is due for each vow.