You should send it back.
The answer is you don't take it. In a personal injury case, it's just not possible to fully assess the nature and extent of an injury after a few hours or a few days. It might actually be an unfair claims practice or potentially even an illegal act for an insurance company to offer you money within a given time period after an accident. My standard advice in these situations is, seek the guidance of an experienced lawyer to look at all of the factors that go into the value of a case and then make an informed decision after you've had a chance to seek counsel.