Should he return money to the person who tried to trick him and defraud him?
Question

Someone wanted to defraud me, so he gave me some money so as to entice me to give him my money [as an investment] so that he could multiply it for me, and I kept the money that he gave me. Is this money haram?
Praise be to Allah.With regard to the money that you received from this person, if it was a gift, then you do not have to return it.
But if it was a loan, or something entrusted to you for safekeeping, or a partnership, then you must return it to him, and it is not permissible to keep it on the grounds that he wanted to defraud you, because it is haram to take people’s wealth unlawfully and it is haram to take wealth unless it is given willingly by its owner.
Allah, may He be exalted, says (interpretation of the meaning):
{ O you who have believed, do not consume one another's wealth unjustly but only [in lawful] business by mutual consent} [An-Nisa’ 4:29].
And the Prophet (blessings and peace of Allah be upon him) said: “It is not permissible to take the wealth of a Muslim except with his willing consent.” Narrated by Ahmad (20172); classed as sahih by al-Albani in Irwa’ al-Ghalil (1459).
And the Prophet (blessings and peace of Allah be upon him) said: “Your blood, your wealth and your honour are sacred to one another.” Narrated by al-Bukhari (1739) and Muslim (1679).
And the Prophet (blessings and peace of Allah be upon him) said: “Render back the trust to the one who entrusted it to you, and do not betray the one who betrayed you.” Narrated by Abu Dawud (3534); classed as sahih by al-Albani in Sahih Abi Dawud.
It says in ‘Awn al-Ma‘bud (9/327): To sum up, the trust can never be betrayed, because the one who entrusted it to you is either honest or dishonest, but whatever the case, you must not betray it. End quote.
So what you must do is return the money to its owner, so long as it was not a gift.
Even if it was a gift, but you found out his intention, then it was not a true gift and was not given for the sake of Allah or because of friendship between you, or from other noble reasons. So the best is for you to refrain from keeping his money, and to return it to him, and if you tell him what you found out and denounce his actions, that will be better.
And Allah knows best.