Praise be to Allah.
Firstly: You should realize that the mercy of Allah is greater than anyone can imagine. Allah says (interpretation of the meaning):
“... and My Mercy embraces all things.”
And it was narrated from Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: “Allah has one hundred degrees of mercy, of which He sent down one among the jinn, mankind, the animals and the vermin, by virtue of which they show compassion and mercy to one another, and the wild animal shows compassion to its young. But Allah has kept behind ninety-nine degrees of mercy by virtue of which He will have mercy on His slaves on the Day of Resurrection.” Narrated by Muslim, 2752.
A person repented from killing one hundred souls, as was narrated in the saheeh hadeeth (authentic narration) from the Messenger of Allah (peace and blessings of Allah be upon him) (narrated by Muslim, 2766), and stealing is not more serious than killing. So congratulations for your repenting and turning to Allah.
Secondly: The scholars stated that there are three conditions for repentance: giving up the sin, regretting what is past, and resolving not to go back to it.
You should understand that rights are of two types: the rights of Allah and the rights of other people. The rights of Allah are waived when one repents, except in the case of financial dues such as zakah (poor-due), kaffarah (expiation) and vows – according to the correct opinion.
See Rawdat al-Talibeen, 11/246; Kashshaf al-Qina’, 2/257.
With regard to the rights of other people, the scholars added a fourth condition to repentance, which is that people’s rights should be restored to them, and that repentance is not sufficient to waive any of the rights owed to others, rather the rights of the one who has been wronged must be restored.
This was mentioned by Ibn Qudamah, al-Nawawi, Ibn al-Qayyim, Ibn Hajar and others. See: al-Mughni, 14/193; Rawdat al-Talibeen, 11/245-246; Madarij al-Salikeen, 1/396; Fath al-Bari, 11/104.
Thirdly: What you mention about not being able to return the money is no justification for taking this matter lightly. Try to collect the money from halal (lawful) sources such as gifts, work that is Islamically acceptable, and so on. Allah will help you with that if He knows that you are sincere, as He says (interpretation of the meaning):
“And whosoever fears Allah and keeps his duty to Him, He will make a way for him to get out (from every difficulty).
And He will provide him from (sources) he never could imagine.”
Then give this money to its rightful owner and tell them frankly, if that will not cause embarrassment or lead to harmful consequences; if being frank will cause too much embarrassment, then you can give it to them in a way that will not cause any trouble between you and them. See question no. 45016.
If you cannot return the money because the amount is too great and you cannot afford it, then you can ask the owners to forgive you. Seek the help of Allah in this and put your trust in Him.
“And whosoever puts his trust in Allah, then He will suffice him.”
If you cannot do that either because you do not know who the owner is, for example, then we hope that Allah will forgive you.
Al-Nawawi (may Allah have mercy on him) said: If the sin has to do with financial duties, such as withholding zakah, seizing property by force and crimes against people’s wealth, then as well as that (repentance) the duty must also be discharged, such as paying zakah, returning people’s wealth to them if it is still present or paying them the equivalent otherwise, or asking the person to whom it is owed to let him off. If the person has died, then it should be given to his heir; if he does not have an heir or he cannot be found, then the matter should be referred to a qadi (judge) of good character.
If this is not possible, then it should be given in charity to the poor with the intention of also paying it back if he (the one who took it) finds him; if he is poor then he should have the intention of paying it back if he can. If he dies before he can pay it back, then there is the hope that Allah will forgive him by His grace.
Rawdat al-Talibeen, 11/246.
And Allah is the Source of strength.