The terms allow Leucadia - parent of investment bank Jefferies - to force the sale of FXCM after three years, and receive half the proceeds beyond repayment of the $300m (£197m) loans and fees.
The two-year loan made to FXCM will have an interest rate beginning at 10 per cent, but this could rise as high as 17 per cent.
"[Leucadia National's] financing and ongoing support will enable us to continue to provide the highest quality service to our customers and act as the leading online provider of foreign exchange trading and related services to retail and institutional customers worldwide," Drew Niv, chief executive of FXCM, said.
Leucadia threw FXCM a lifeline after the US's largest retail currency broker was hurt by the Swiss central bank's shock decision to scrap its four-year old currency ceiling - prompting chaos in currency markets.
FXCM said the soaring Swiss Franc led to customers owing it about $225m, meaning it could be in violation of its capital requirements.