Praise be to Allah.
Because you were at fault and are the one who caused this accident, then you must bear responsibility for it in full. If the other party wants to let you off and forgive you, then he has the right to do that. It is his right: he can waive liability or he can demand compensation.
But he does not have the right to deceive the insurance company and make them think that he was the one who caused the accident, so that they will bear the costs of repair, because this is lying, cheating and deceit, and it is contrary to the conditions of the contract between him and this company.
Based on that, you have to return this money to the insurance company, because you took it unlawfully. If it is too difficult to return it to them for any reason, then you must give it in charity to the poor and needy.
The fact that you are not well off in contrast to the wealth of the other man is not an excuse or justification for obtaining the money unlawfully.
See also the answer to question no. 133116.
And Allah knows best.