What is the ruling on money taken by a person as a gift from a colleague, which the colleague repeatedly refused to take back, and then demanded it back after two years on the pretext that the former had lied to him about his financial situation?

If you acquired money through deception by claiming you are unemployed, it is not initially permissible for you. However, if you inform him of the truth and he refuses to take it back, then there is no harm in you benefiting from it. If he demands its return, then tell him that one who takes back his gift is like a dog that returns to its vomit. If he insists on its return after that, then return the money to him, in accordance with the Hadith: "The example of one who takes back what he has given as a gift is like the example of a dog that vomits and then eats its vomit."