What is the ruling on someone who seized another person's effort, attributed it to himself, and earned money from it, then repented and felt remorse, and what should he do to expiate his sin?

What this person did, infringing upon the rights of others and stealing their work, is not permissible Islamically. His repentance to Allah Almighty absolves him of Allah's right, but the rights of people are based on mutual consent and forgiveness. Therefore, he must seek forgiveness from the person whose rights he violated and appease him, even if it is with financial compensation. If he cannot do so directly or fears discord, he can deliver what is rightfully due through indirect means.