WILMINGTON, Del., Oct 4 (Reuters) - A Delaware judge should order Elon Musk to repay $9.4 billion to Tesla Inc (TSLA.O) for the benefits he received by pushing the electric vehicle maker in 2016 to buy an ailing solar company he partly owned, according to Tesla investors in a court filing late Friday. The investors said a "simple and practical" way to undo the benefits they say Musk improperly received from the $2.6 billion deal for SolarCity would be to order the chief executive to return the Tesla shares he received for his stock in the rooftop solar company. Musk received 2.4 million Tesla shares, which have grown to 12 million shares due to stock splits and are currently worth about $9.4 billion. The investors said Musk, one of the world"s richest people, could also repay the value of the stock to the company. Tesla stock was up 2.5% in early Monday trading at $792.76 per share. The lawsuit was brought by union pension funds and asset managers. They allege in their post-trial brief that Musk did not recuse himself from the deal negotiations and never disclosed SolarCity was nearly out of cash. Musk said in his own post-trial brief filed late Friday that SolarCity was the culmination of a decade-long plan to create a vertically integrated clean energy company offering roof-top solar generation combined with Tesla"s battery storage and vehicles. He said the deal was struck at a fair price and approved by 85% of Tesla shareholders. read more The briefs follow a two-week trial in July in Wilmington, Delaware. read more The judge, Vice Chancellor Joseph Slights, must still answer a key question that could determine the outcome of the case - whether Musk is a controlling shareholder despite owning a minority stake in Tesla. read more Slights has scheduled post-trial arguments for January and a ruling would likely take place months after that.