He wanted to travel to Rolleston, he said, and the price changed to $300.
He stopped at a cash machine and withdrew the money.
Somewhere around Templeton, just south of Christchurch, they stopped on the roadside and had sex in the backseat of his car, Marong said.
Afterwards, Duckmanton wanted to return to Manchester St, saying the "job is done".
But Marong wanted to continue to Rolleston, and he says there was an argument.
"She started yelling at me," Marong said.
He said "that voice, that screaming, that yelling was agitating me" and the dispute "triggered something very serious".
"The only method I could use to stop it was to compress her neck," he said.
Asked by defence counsel Jonathan Krebs if he meant to kill her, Marong replied, "I have no motive to cause any harm, I just did it to silence her."
Duckmanton's family in the public gallery were upset by Marong's description of events. Marong himself used a tissue to wipe his eyes.
By the early morning of May 15, Marong realised that Duckmanton was dead.
But he was so "disrupted" that he never paid her any attention because "her mouth and eyes were open" which reminded him of "the animals that normally get slaughtered at my workplace".
While walking to Rolleston McDonald's that day, he came up with the idea of setting her body on fire.
"I couldn't resist myself from doing it," he said.
He claims to have no clue why the sheep's tongue he had cut from a slaughtered animal ended up at the scene of the fire on Main Rakaia Rd near State Highway One.
It wasn't until the following Saturday night during a dinner with friends that he first comprehended that he had "took a human life".
The Crown claims its case against the butcher originally from Gambia is "overwhelming".
It claims DNA samples taken from Duckmanton, and from samples found where her body was dumped, allegedly belong to Marong.
Earlier, Marong says that in January 2016, he stopped taking his diabetes medication and soon found himself "going off track".
"I was not the same Sainey," he said.
Marong says amnesia created negative thoughts, an agitation and "impulsivity" that the only way to "dilute" was either seeing or sleeping with prostitutes.
In early 2016, he became "hypersexual", Marong claims, and going into the red light district and just seeing the girls "made me feel comfortable".
Sometimes he would travel from his home in the suburb of Ilam to Manchester St 10 times a day.
Asked by defence counsel Jonathan Krebs how many times he paid for sex a week, Marong was unsure. "So many times," he said.
He said it was "not normal" but reflected his "delusional state of mind" where he felt paranoid and suffering mental illness. His "illness" caused him to have thoughts of violent behavior, Marong said.
Marong's internet browsing history included Christchurch sex workers, a Google search of 'What chemical do kidnappers use', chloroform, necrophilia and local graveyards.
Krebs asked if he could explain the searches and Marong replied that insomnia had caused his brain to generate delusional thoughts.
"I was living in a whole new world. I was deluded, I needed to know many things."
The trial, before Justice Cameron Mander, continues.
