All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
May Allaah reward you for your enthusiasm in finding out what is lawful, refraining from what is prohibited and repenting from it. We ask Allaah, The Almighty, not to let your heart deviate after He has guided you to the sound faith; as Allaah hears and answers.
As for your question, the answer is that you have to give back the money that you have stolen to its owners if you know them. If you do not know them, then you should give it in charity (on behalf of its owners). If this entails that you sell the belts you have, then you are obliged to sell them; otherwise, you are not obliged to do so.
The same ruling applies to the money given to you and which you believe is most likely stolen. You have to give it in charity on behalf of its owners if you do not know them, which is implied in your question.
This is regarding the original sum of money taken unjustly. As for what you have earned from trading with that unlawful money, it is like the profit which results from the unlawfully acquired money. Does it belong to the usurper or for the owner of the money? Scholars held different opinions regarding that and the strongest opinion is what Shaykh Al-Islam Ibn Taymiyyah adopted, namely that it should be divided between the two equally in half as we have previously pointed out in the Fatwa 101322. Accordingly, the original owners of the money will get half of the profit as a result of investing their money and you will get the other half as a result of your work and effort. So, you can benefit from that half in trade or otherwise.
You also should know that whoever gives up a thing for the sake of Allaah, Allaah will give something better in return. Allaah, The Almighty, says (what means): {…And whoever fears Allaah - He will make for him a way out. And will provide for him from where he does not expect…} [Quran 65:2-3]
Abu Umaamah narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: “The Holy Spirit (Jibreel [Gabriel]) has revealed to me that no soul will die until it has completed its appointed term and received its provision in full, so fear Allaah and be moderate in seeking provision, and none of you should be tempted to seek provision by disobeying Allaah if it is slow in coming to him, for that which is with Allaah can only be attained by obeying Him.” [Abu Nu‘aym]
Accordingly, you are obliged to get rid of the stolen money and half of its profits and to be careful to free yourself of liability.
In principle, this is how to repent from the wealth that one wrongly acquires without knowing its owners. However, if you are in need of that money, it is permissible for you to take as much as you need; Shaykh Al-Islam Ibn Taymiyyah said: "If a prostitute or wine seller repents, and they are poor, it is permissible that some of this money be given to them, as much as they need. If he is able to engage in trade or in a profession such as weaving or spinning, he should be given enough to form capital for him."
And if you give it in charity after Allaah, The Almighty, makes things easy for you, that would indeed be great.
Allaah Knows best.