How can I restore people's rights after I bought a property for my friend at a low price and took an increase from him without his knowledge, and now I am unable to face him or return the amount?

It is not permissible for an agent appointed for buying and selling to increase the purchase price without the principal's knowledge. This increase must be returned to its owner. This money remains a debt in the agent's liability if he is insolvent. It is not necessary to inform the principal about the increase; rather, returning it by any means is sufficient. Ibn 'Uthaymeen said: "Whoever has wronged people, if he repents to Allah, must return the wrongs to their rightful owners... He can cleverly achieve this by, for example, writing a letter without mentioning his name and sending it to the owner of the stolen item along with the stolen item, or its value if the item itself is unobtainable, stating in the letter: 'This is yours from a person who wronged you concerning it and has repented to Allah.'"