The employee made the claim after a dispute at Ms Spears’ home on Monday.
The Ventura Country Sheriff’s Office said it will send its inquiry findings to prosecutors for consideration of misdemeanour battery charges.
The singer’s lawyer denied the allegation, calling it “overblown”.
Misdemeanour battery in California is a criminal offence that involves any intentional and unlawful physical contact with someone else.
Ms Spears, 39, is alleged to have slapped her employee’s mobile phone out of their hands during the encounter.
In a statement emailed to the BBC, her lawyer Mathew S Rosengart said it was “nothing more than a manufactured ‘he said, she said’ regarding a cell phone, with no striking and obviously no injury whatsoever.
“Anyone can make an accusation, but this should have been closed immediately,” he said.
“To its credit, the Sheriff’s Office itself has acknowledged that the incident was classified as a ‘very minor misdemeanour…’ and confirmed ‘there were no injuries’.”
Ms Spears hired Mr Rosengart last month as she seeks to end her father’s hold over her life and finances, 13 years after he took legal control when concerns were raised over her mental health.
Last week, Jamie Spears agreed to step down as his daughter’s conservator, pledging to participate in an “orderly transition” to a new legal arrangement.
Ms Spears, who has called the conservatorship “abusive”, has filed a petition to stop her father controlling her estate in July and has said she will not perform again while he remains in the role.