A man who was arrested and charged with robbing the driver of a taxi that he boarded has been acquitted after a trial.

In a written judgment released this week, a district judge said that he found the testimony of cabby Foo Boon Thye "less then impressive" as he had "materially contradicted his evidence about how he was allegedly robbed" of $500.

On the night of Sept 18, 2014, Mr Foo, 41, stopped his empty taxi in Jalan Bukit Ho Swee to return a phone call. Suddenly, the front passenger door opened and Mr Lim Poon San, 42, got in.

The cabby claimed that Mr Lim robbed him.

At first, he claimed that Mr Lim threatened to poke him with a sharp, pin-like object. Afraid, Mr Foo handed over $500 to Mr Lim.

But when cross-examined on the stand, the cabby admitted that Mr Lim never threatened him with such an object. Instead, he said that he handed over the money - two days' earnings - because Mr Lim uttered threatening words.

Mr Foo said he felt threatened because after Mr Lim spoke to him, the latter put his hand into the pocket of his trousers. The driver presumed that Mr Lim was trying to take something out.

After Mr Foo changed his version of events, the prosecution amended the charge to one of robbery by uttering the threatening words: "You don't want to give me, right? You don't want to give me right? Okay."

However, Mr Lim denied robbing the cabby or threatening him in any way.

He told the court he lived nearby and got into the cab to ask for a free ride to the Institute of Mental Health (IMH) as he was feeling depressed, but he was rudely rebuffed by Mr Foo.

Mr Lim, who has a history with the IMH, said he showed the cabby his IMH report and asked for help to get to the hospital as he did not have enough money for the fare.

He said Mr Foo scolded him and told him to go away.

In his judgment, district judge Eugene Teo said the prosecution's case rested on the court finding that Mr Foo's testimony was "unusually convincing".

"We instead have a witness who could not even maintain a consistent stance on the central point in dispute: That is, on how the alleged robbery took place," he said.

The judge said he could not discount the possibility that the robbery charge was being "trumped up" by Mr Foo.

Mr Lim's lawyer, Mr Gino Hardial Singh, assigned under the Criminal Legal Aid Scheme, had made the suggestion - which was denied by Mr Foo - that the cabby made up the allegations to get back at Mr Lim for hurling vulgarities at him and slamming the taxi door.

The prosecution has filed an appeal against Mr Lim's acquittal.