Innisfree M&A claims it was hired to facilitate communications between Twitter and shareholders last year.
After Musk's acquisition was finalized, Innisfree sent Twitter an invoice for $1.9 million, which was "processed" a month later.
The suit claims Twitter has not responded to Innisfree's invoice and letters since December.
A lawsuit filed Friday in New York State Supreme Court by Innisfree M&A Inc. alleges that Twitter never paid the advising firm over $1.9 million for services the company performed during the sale of Twitter to Elon Musk last year.
In the suit, Innisfree alleges that it was hired last May to help facilitate communications to shareholders about the September meeting in which they would vote on Musk's potential purchase of the social media giant.
After the vote to finalize Musk's purchase was complete, Innisfree's work was done and a few weeks later an invoice was sent to Twitter for $1,902,788, according to the New York Times. Innisfree alleges that it submitted the invoice in September, and was informed in October that it had been "successfully processed," though the firm had still not received payment.
After following up twice in December because no payments had been made, Innisfree said that Twitter has not contacted the firm since, leading it to file suit over the alleged breach in their agreement, according to the Wall Street Journal.
The lawsuit is the latest in a string of recent legal and regulatory challenges filed against Twitter, including accusations of unpaid rent, and another unpaid bill from a private jet company, among others.
The landlord of Twitter's San Francisco office claimed in a suit filed last month that the company failed to pay rent in December and January, and therefore owed several million dollars. Another San Francisco landlord sued over about $130,000 in unpaid rent in early January, and a real estate company belonging to King Charles III also sued Twitter over unpaid rent at a London office.
Another $2 million in consulting fees is the subject of a lawsuit filed last month by Charles River Associates. It alleges that the company provided various services advising Twitter when it was still suing Musk to force through the acquisition after the billionaire tried to renege. The suit was dropped when the sale went through late last year, but Charles River Associates said in its suit that it was never paid.
Insider previously has reported that Musk's drastic cost cutting measures - including layoffs and the halting of rent payments - were designed to free up money to begin paying interest on the $13 billion in debt he took on to complete the acquisition.
A request for comment from Twitter was unanswered Sunday afternoon.