Racing announced a deal with Matthaus two days ago at a press conference but the 48-year-old, who has coached teams in Hungary, Brazil and Israel recently, insisted no contract had been signed yet.
One of Argentina's so-called 'Big Five' clubs with seven league title to their name - though only one since 1966 - Racing are joint bottom of the Apertura championship on five points from 10 matches and yet to win this season.
"The contract is 80 percent finalised. There are still issues to be clarified," Matthaus told Germany's Bild newspaper, adding that modeling work for his 21-year-old wife Liliana was also a factor.
"I have to talk with her first. She will only come if she can work as a model there," he said.
Racing chairman Rodolfo Molina told reporters on Saturday a deal was done.
Matthaus, who captained West Germany to World Cup victory in 1990 and had a successful playing career at Bayern Munich and Inter Milan, has enjoyed relatively little success as a coach.
His only title was with Austria's Salzburg in 2007 in conjunction with Giovanni Trapattoni, his former coach at Inter, but he was fired soon afterwards.
