Last year, Microsoft co-founder Bill Gates bought shares in the company with Soros making a similar move back in January. FCC's shareholders then approved a 1 billion euro ($1.24 billion) capital increase last Thursday in the hope of further strengthening capital, reducing debt and improving financial results.
It wasn't long before the buyers came flooding in, with FCC in talks earlier this week with the Soros Fund Management LLC. However, an announcement on Monday confirmed that the discussions had finished without a deal, according to Reuters.
Read MoreWhy Soros and Paulson's bet on Spain could pay off
Instead, FCC then turned to Slim and his company, Inmobiliaria Carso, with the Mexican becoming the largest investors in the Spanish builder, according to Reuters, with a 25.634 percent stake. The investment is worth around 650 million euros, according to the reports.
Meanwhile, Spanish billionaire businesswoman and philanthropist Esther Koplowitz will reduce her stake, according to the newswire, and will hold a 22.433 percent chunk after originally having over 50 percent.
FCC were not immediately available for comment when contacted by CNBC.
Read MoreSpain's FCC says no deal with Soros, in talks with Mexico's Carso