A judge agreed to dismiss allegations by one of three accusers in the case, Lucia Evans.
Weinstein appeared before New York Judge James Burke as his lawyers tried to have some or all of the charges he faces dismissed.
The woman, prosecutors said, told the detective in February that Weinstein had offered them money to flash their breasts during the restaurant encounter.
An attorney for the Harvey Weinstein accuser whose case has been dropped says it does not "invalidate the truth of her claims".
"According to the Witness.the Complainant [Evans] told the Witness that.the Complainant had gone to the defendant's office, where the Defendant told her, in substance, that he would arrange for the Complainant to receive an acting job if she agreed to perform oral sex upon him".
The friend told prosecutors that Evans admitted she performed the sex act and during the conversation was "upset, embarrassed and shaking".
According to the letter, the detective told the witness she had no obligation to cooperate with prosecutors and "less is more". Five other charges, involving alleged assaults of two other women, remain in the case in Manhattan criminal court.
"In short, your honor, we are moving full steam ahead", she said.
Manhattan District Attorney Cyrus R. Vance Jr. has already been fiercely criticized for declining to prosecute Weinstein when an Italian model accused him of grabbing her breasts in 2015.
"People always ask why don't sexual assault survivors come forward, this is why".
"Nor does it reflect on Lucia's consistent allegation that she was sexually assaulted with force by Harvey Weinstein". "It only speaks volumes about the Manhattan DA's office and its mishandling of my client's case".
An attorney for Evans told the Associated Press the dropping of the count "says nothing about Weinstein's guilt or innocence".
"I think the case against Mr Weinstein is, in my view, not sustainable", Brafman added, saying he would submit additional motions in November.
He walked into the criminal court in downtown Manhattan dressed in a dark suit, tie and white shirt for the brief hearing that scored at least an initial win for his defence attorney Ben Brafman. "If you have a person willing to commit perjury in the grand jury, that is as serious as the crime of sexual assault because it undermines the fairness of the process for all of us".
Weinstein's lawyer Benjamin Brafman maintained the jettisoned count is a sign the entire case is built on a weak foundation, one he and the legal team intend to spend the next several weeks undermining.
Prosecutor confronted the detective, who admitted he'd failed to turn over the damning details, but denied discouraging the witness from cooperating.
Mr Brafman also said he would investigate what he said had been "perjury" before the grand jury which indicted Mr Weinstein, as well as the conduct of a NY police officer who had worked on the case.
Weinstein has pleaded not guilty to all charges and is free on $1 million bail.
The numerous sexual misconduct accusations made against Weinstein, namely those made out of court, had raised awareness of the #MeToo movement and increased public debate about the issue of consent.
The collapse of part of the case against him could mean trouble for the prosecutor.