The Believe hitmaker had sued Mary Bono and accused her of withholding payments from songs within the Sonny + Cher catalogue.
In November, in a final judgement, US District Judge John A. Kronstadt formally lodged his 2024 ruling that found Mary was not allowed to use the federal Copyright Act to reclaim the 50 per cent share of Sonny's royalties, which Cher had been granted in their 1978 divorce settlement.
The 79-year-old singer has now filed a new motion asking Mary to foot the bill for her legal costs because she tried to "misuse" the Copyright Act in order to illegally "obtain a windfall" at her expense, and her lawyers argue the defendant had "dragged this case out for five years by taking patently unreasonable positions.".
Documents obtained by Rolling Stone magazine stated: "Defendant argued, repeatedly and frivolously, that a federal statute that expressly states it does not affect state-law rights somehow extinguishes those state law right. Cher prevailed completely."
Cher claimed she was forced to run up a bill of $US1,023,605.50 ($A1.53 million) with her lawyers to overcome Mary's "improper attempt at termination."
While Mary's lawyers - who lodged an appeal in December - have yet to respond, they previously requested neither party be allowed to recover legal costs.
A hearing on the matter will take place on February 23.
Mary had sought to terminate the rights to songs including I Got You Babe and The Beat Goes On, along with Cher's 50 per cent share of the musical recordings she made with Sonny, but the judge ruled California contract law, which oversaw her divorce agreement, came ahead of the copyright termination power of the federal Copyright Act.
And according to Rolling Stone, the judge also ruled Cher retains her right to have her composition and record royalties paid directly to her, despite selling the rights to Iconic Artists Group in 2022.
Sonny died in a skiing accident in 1998 aged 62, with his share of the publishing revenue passing to his heirs, and the legal case came about after Mary exercised a feature of copyright law that allows songwriters and their heirs to win back rights they have signed away in 2016, as she argued she now owned her late husband's publishing rights.
Royalties were then withdrawn from the Strong Enough hitmaker in 2021, prompting her to take legal action and her team insisting the termination clause used by Mary was "wholly inapplicable" to the royalty agreement set out in the former couple's divorce settlement.
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