Days before his inauguration in Washington in 2017, Donald J. Trump had a debt to settle in New York: a payout to his fixer, Michael D. Cohen. They met on the 26th floor of Trump Tower, Mr. Cohen says, and struck a $420,000 deal.

Seven years later, Mr. Trump’s criminal trial in Manhattan hinges on that fleeting encounter, which is both critically important and completely in dispute. Mr. Cohen says he paid off a porn star at his boss’s behest, and in that meeting, he and Mr. Trump settled on a plan to repay him and conceal the reimbursement as legal expenses. Mr. Trump says his former fixer is a liar.

But prosecutors say there was a third man in the room that day: Allen H. Weisselberg, Mr. Trump’s moneyman, the keeper of the balance sheet. And he is not saying anything at all.

Prosecutors never called Mr. Weisselberg to testify, because, although he knows the truth, he has not always told it. He is serving time in the Rikers Island jail complex after pleading guilty to perjury in an unrelated civil case involving Mr. Trump, the man he served for nearly half a century.

The defense did not call Mr. Weisselberg either, nor did Mr. Trump take the stand in his own defense. And for weeks, Mr. Weisselberg’s absence has loomed large over Mr. Trump’s case, the first criminal trial of an American president.

In closing arguments on Tuesday, his lawyers are expected to cast Mr. Weisselberg as the prosecution’s missing piece, an elusive central player in the case. They are likely to underscore for the jury just how much of the case depends on the word of Mr. Cohen, whose testimony provided the only evidence that Mr. Trump had direct knowledge of the plot to disguise the hush-money reimbursement as legal fees.