The world's second wealthiest man has bought 6 percent of heavily indebted Spanish construction company FCC for €113.5 million ($155 million).

Although FCC lost around 80 percent of its share value during Spain’s property crash, a €16.9 billion ($22.5 billion) contract to build a metro in Riyadh in Saudi Arabia has got the company back on its feet.

It also represented "the biggest international contract in the history of Spanish construction", FCC said in a statement on Monday.

As for Bill Gates’ relationship with Spain, the software magnate turned philanthropist met with Spanish PM Mariano Rajoy in 2012 to ensure Spain would continue participating in humanitarian projects despite the crippling crisis.

Newspaper El Economista is now saying Gates’ investment in FCC is a way of returning the favour for Rajoy’s commitment to the billionaire’s charitable cause.

Gates was also reported as saying he found it “strange that wages in Spain hadn’t fallen with all that unemployed labour”.

“I am not a technocrat who has closely studied the situation of Spain,” he told Spanish newspaper El País in 2012.

“But there is probably a once-in-a-lifetime opportunity to take those difficult decisions to create the flexibility typical of the labour markets in Ireland, the United Kingdom and the United States.”