The Archdiocese of Los Angeles and the Sisters of the Immaculate Heart of Mary are going head to head in court on Thursday to argue who should be able to sell a former convent.
The nuns sold the property, one of the most exclusive in the state, to Los Angeles restaurateur Dana Hollister for around $15.5 million (around £9.9 million).
Archdiocese officials claim this was unlawful and the property had been sold by it to singer Katy Perry.
It has now accused Ms Hollister of taking advantage of the nuns in order to drive the deal.
It is seeking to dismiss the sale reached in June so the home can be sold to Perry, famous for hits like Firework, Roar and Hot 'n' Cold.
Officials have claimed the deal is invalid and have questioned how Ms Hollister was able to buy by paying a deposit of just $44,000 (£28,000).
Dana Hollister hit back at the archdiocese and said the nuns had the right to choose who bought the convent. Speaking to the Washington Post, she said: "Those ladies know what they're doing.
"They are by no means fragile.
"All of these women who have worked in the religious world have absolutely no rights, and these ladies are fighting for them."
The nuns have said selling to the world famous star would 'violate' their pledge to the Catholic Church.
Lawyer for the Archdiocese, J. Michael Hennigan, said: "What Dana Hollister did in this transaction was take their principle asset, give them next to no money, with a flaky promise to pay in three years."
Katy Perry has so far not commented on the court battle but has previously said she wants to buy the Los Feliz hilltop mansion for her mother and grandmother.
The singer's mother and father are both pastors and before entering pop music Perry recorded a worship album.
She met the nuns in May to attempt to convince them to approve the sale.
It's reported she sang the gospel hymn Oh, Happy Day and showed the sisters a tattoo of the word 'Jesus'.
Sister Rita Callanan told the Los Angeles Times she searched the internet for Miss Perry: "I found her videos. I wasn't happy with any of it."