Praise be to Allah.
In order for repentance from sins having to do with people’s rights to be accepted, it is stipulated that the rights be restored to those who were entitled to them, because the Prophet (blessings and peace of Allah be upon him) said: “Whoever has wronged his brother, let him ask him for forgiveness, because there [in the hereafter] there will be no dinar or dirham. [So let him do that] before some of his hasanaat (good deeds) are taken from him and given to his brother, and if he has no hasanaat to his credit, some of his brother’s sayyi’aat (bad deeds) will be taken and added to his burden.” Narrated by al-Bukhaari (6534).
If he took some of his wealth from him by force or by means of a trick, he should ask him to forgive him, or return that wealth to him by any possible means, and it is not stipulated that he should inform him of that. If he has died, he should give it to his heirs.
If he is unable to find the one whom he wronged, he should give the wealth in charity on his behalf.
If he is unable to return the wealth, and he is not able to ask the one whom he wronged to forgive him, let him repent to his Lord, and perhaps Allah will enable him to settle the matter on the Day of Resurrection.
An-Nawawi (may Allah have mercy on him) said in Rawdat at-Taalibeen (11/246): If some financial right is connected to this sin – such as withholding zakaah, seizing wealth by force and other transgressions against people’s wealth – then in addition to repenting, he must also absolve himself of responsibility by giving zakaah and returning people’s wealth, if it still exists, or replacing it, if it no longer exists, or he should seek forgiveness from the one to whom it is owed, so that he may forgive him.
He has to inform the one to whom he owes that wealth, if he is not aware of it, and make sure that it reaches him, if he is elsewhere and he took the wealth from him by force in that place. If he has died, he must give it to his heirs; if there is no heir and he has disappeared, he should give it to a qaadi of good conduct who is religiously committed, and if that is not possible, he should give it in charity to the poor, with the intention of repaying the original owner if he ever finds him.
If he cannot afford to do that, he should intend to pay it when he can afford it. If he dies before he is able to pay it, there is the hope that he will be forgiven by the grace of Allah, may He be exalted.
An-Nawawi said: I say: according to the apparent meaning of the sound reports, the individual is responsible for wrongdoing, even if he died unable to pay it back, if he borrowed the money for sinful purposes.
But if he borrowed money in a situation in which it is permissible to borrow, and was not able to pay it back before he died, or he caused damage to something accidentally and was not able to pay liability before he died, then what appears to be the case is that he is not responsible for that in the hereafter, because there was no sin on his part, and there is the hope that Allah, may He be exalted, will compensate the one who was wronged.
With regard to backbiting, if it did not reach the person who was the target of the backbiting, then I have seen in the fatwas of al-Hanaati that it is sufficient to feel remorse and seek forgiveness. If news of it did reach the person who was the target of the backbiting, then what he should do is go to that person and ask him to forgive him. If that is not possible, because he has died or because he is too far away, then he should ask Allah, may He be exalted, for forgiveness, and there is no need to ask that person’s heirs for forgiveness. This is what al-Hanaati said. End quote.
Financial rights must be restored to the one who was wronged, but in the case of intangible rights, it is sufficient to feel remorse and seek forgiveness, if news of that did not reach the one who was wronged.
With regard to what you mentioned about reducing the rise in salary of that employee, or not giving him the promotion that he deserved, this is a transgression against his tangible rights, by depriving him of money that he deserved; it is also a transgression against his intangible rights, by preventing him from getting a promotion that he deserved.
Based on that, with regard to his financial rights, you have to ask him to forgive you, or give him the money which is equivalent to what he was deprived of because of your wrongdoing.
You can seek the help of someone who will intercede with the one who was wronged and ask him to forgive you.
If you are not able to do either of these two things, then you should feel a great deal of remorse and ask Allah to forgive you, and ask Him, may He be exalted, to settle the matter on your behalf on the Day of Resurrection.
With regard to his intangible rights, if he was unaware of your wronging him, then it is sufficient to feel remorse and ask Allah for forgiveness. If he was aware of it, then you must ask him to forgive you, so long as you do not fear greater mischief if he finds out about it.
We ask Allah to accept your repentance, help you to fulfil your duties and help you to obey Him.
And Allah knows best.