Patel did not negotiate a deal with DOJ. He took the 5th and Judge Howell ruled that he was justified in doing that, so DOJ is compelling him to testify. In order to get the judge to issue an order compelling his testimony, they had to give him

LIMITED immunity. This means they can still charge him, but they can't use his testimony or any information that he provides in this case against him. They are also free to charge him for any other crimes that he has committed. https://theguardian.com/us-news/2022/nov/02/trump-advisor-kash-patel-immunity-mar-a-lago-documents