Impact of Trump CFO Allen Weisselberg's immunity deal

Immunity isn't simply given as a trade for testimony. Because a person has their 5th amendment rights: which would allow them to refuse to testify to anything at all, the prosecutor's answer is to strip the witness of their 5th amendment defense by immunizing them.  With immunity, the witness now cannot invoke 5th amendment rights because they cannot be held criminally liable for the testimony which might incriminate them.