At Nottingham Crown Court, Richard Wakerley QC, repeatedly asked Sams, 51, to name the friend. But the defendant refused until he had first proved his own innocence.
John Milmo QC, for the defence, concluding his examination in chief, asked: 'One final question. Are you prepared to name your friend?'
Sams replied: 'Not at this moment, no.'
Mr Wakerley then stood up to begin cross-examination and asked: 'When will you?'
Sams said: 'When I have proved I did not kill Julie Dart, after the conclusion of this trial.'
Mr Wakerley asked: 'Why then?' Sams replied that because then the police would have to believe there was someone else involved.
Mr Justice Judge told Sams: 'If you are not prepared to identify this man in court, the jury may find it hard to believe he exists at all. Are you still not prepared to name him?'
The defendant answered: 'No.'
Mr Wakerley then asked Sams what he thought when his friend told him he had killed someone.
Sams replied that he believed he was simply helping his friend cover up the crime.
Asked whether he was worried that he would be considered an associate in the crime, Sams said: 'No, I didn't think I would be implicated in murder just by writing some letters.'
Sams, a tool repairer of Barrel Hill Road, Sutton-on-Trent, Nottinghamshire, has admitted kidnapping Birmingham estate agent Stephanie Slater, unlawfully imprisoning her and demanding pounds 175,000 with menaces from her then employers.
He has denied kidnapping and murdering Leeds prostitute Julie Dart. The trial continues on Monday.