The duo was said to have had an argument before the 23-year-old got a knife and stabbed Haruna to death.
The incident happened around 6:00 a.m. on Tuesday at their home in the Surulere area of Lagos State.
She was subsequently arrested by the police.
The police spokesman, DSP Bala Elkana, confirmed the incident.
He said, “The couple were not married; they lived together for three years on Tejuosho Avenue, Surulere Lagos, and have a baby who just turned one.
“An argument ensued between them which turned violent and the woman used a kitchen knife to stab the man in the back which led to his death.
“The woman accused the man of refusing to release money for their daughter’s birthday party.”
Elkana said homicide detectives from the State Criminal Investigation and Intelligence Department, Yaba, have taken over the investigation.
He noted that the woman had confessed to the crime and she would be charged to court for murder.