Is the money taken by someone from a swindler, who gave it to him to tempt him into investing his money, unlawful (haram)?

If the money was a gift, there is no obligation to return it. However, if it was a loan, a deposit, or a partnership, it must be returned to its owner. It is not permissible to take it under the pretext of an attempted fraud, for Allah the Almighty says: (And do not consume one another's properties unjustly but only [in lawful] trade by mutual consent), and for the Prophet, peace and blessings be upon him, said: (The property of a Muslim is not lawful except with his willing consent). Likewise, a trust must be returned, even if its owner is a traitor. It is better to return the money even if it was a gift, if it is known that the intention was not pure.