Constance Meester is suing her famous daughter over claims the actress reneged on an oral agreement to pay her $10,000 (£6,250) every month for life.
But Leighton's attorneys argue no such deal is in place. In a petition filed last week (ends04Sep11), the star's legal team contends the suit should be thrown out since Constance is suing over "a non-existent management agreement".
Leighton sued her mum in July (11) in a bid to have a judge declare there was never any binding contract between them.
The 25 year old also accused Constance of taking monthly payments intended for her sick brother Alexander and using the money to pay for cosmetic treatments.
Her mum launched a countersuit days later.
A hearing on the matter is scheduled for 5 October (11), according to Eonline.com.