Assalaamu alaykum. Is it better to do a part-time job for money or to do it for free? Also, will I be punished if I give wrong advice to anyone, like if I tell him to do the job for free for the reward in the Hereafter? This is my main question: is my advice correct? If not, then what should I do now that I have already given this advice?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

A Muslim is obliged to advise his Muslim brother, and he should advise him sincerely and guide him to what he would love for himself if he were in his place.

The Prophet said, “None of you truly believes until he loves for his brother what he loves for himself.” [Al-Bukhaari and Muslim]

If a Muslim advises his Muslim brother to do something while he knows that something else is better for him, then this is a betrayal and sin.

Al-Qurtubi said in Al-Mufhim, “A person who is asked to give advice must give advice according to what he believes to be the best, as the Prophet said, ‘If a Muslim advises his Muslim brother to do something while he knows that something else is better for him, then he has indeed betrayed him.’ He also said, ‘An adviser (a person who is consulted to give advice) is a trustee.’”

Bareeqah Mahmoodiyyah reads:

“Abu Hurayrah, may Allah be pleased with him, narrated that the Prophet said that the person whose advice is sought is a trustee, that is to say that he is trusted about the matters which he is asked about, so he must not conceal a benefit for the one who sought his advice, and if he conceals a benefit from him, then he has harmed him, while the Prophet said, ‘There should be neither harm nor reciprocal harm.’ Hence, the adviser should only advise as to what he believes to be the best. In Al-Jami', there is an addition: 'If he is consulted for advice, then he should advise in the same manner that he would do himself, and if a Muslim advises his Muslim brother - and it was said: even if the latter did not seek his advice - about a matter while he knows that if he does something else it is better for him, then he has indeed betrayed him.'” [Abridged]

As for the issue of your advice to your friend about that matter, then we do not know what it really is, nor its conditions and circumstances, so it may be a betrayal if you know that it is better for him to work in return for payment and you advised him otherwise, such as you advising him to do so for the interest of the employer, or so that he would not take a salary like you, and so forth, in which case you must repent to Allah of what you have done.

Allah knows best.