Forrest was in the German capital on behalf of FMG subsidiary Fortescue Future Industries to sign a memorandum of understanding with European energy major E.ON.
The MoU set out plans for the companies to partner to provide up to 5 million tonnes of green hydrogen to Europe by 2030.
"On both sides, it will be like a duck swimming," Forrest said during the event.
The two companies' executives would be "really calm", but "underneath, our organisations will be going like crazy, because for us it's a minimum US$50 billion expenditure".
In a clarification issued after a brief trading pause this morning, FMG said it stood by the figure, but clarified that it was a high-level assessment by Forrest of what such a major project might cost.
The company said it was appropriate in the environment the statement was made to provide context and scale of the potential of the MoU.
"Fortescue clarifies that there is no commitment to this expenditure and all such final investment decisions will be at the sole discretion of the Fortescue board," it said in a statement.
Last year, Forrest conceded the company's green energy plans would cost hundreds of billions of dollars over time, but would be "funded by the world's greatest institutions".
FMG said today that there was widespread investor interest in FFI "across the full spectrum of investors ranging from retail, through institutional investors and sovereign wealth funds, and across the capital structure".
"We are working closely with banks, institutional investors, multilaterals and export credit agencies, globally, as they confirm their commitment to supporting project funding for green energy," the company said.
"They are actively seeking to invest and commit the capital required to scale green energy projects and help steer companies along Paris-aligned net-zero pathways."
FMG has committed 10% of net profit after tax to FFI - an estimated $1 billion in FY21.
Yesterday's MoU was signed by Forrest and E.ON chief operating officer Patrick Lammers in the presence of Michael Kellner, parliamentary state secretary at Australia's Ministry for Economic Affairs and Climate Action and Australia's ambassador to Germany Philip Green.
FFI said the deal would help Europe reduce its energy dependence on Russia.
"Green energy will reduce fossil fuel consumption dramatically in Germany and quickly help substitute Russian energy supply, while creating a massive new employment intensive industry in Australia," Forrest said.
"This is a cohesive and urgently needed part of the green industrial revolution underway here in Europe."
FMG CEO Elizabeth Gaines said the company was rapidly establishing the building blocks that would allow it to develop and fully integrate its world-leading green technologies, manufacturing capabilities and green energy generation and distribution.
"Today's important announcement in Europe is not just signalling to the global marketplace that now is the time to make renewable GH2. It is also a decisive step forward in FFI's journey to become one of the world's largest green energy producers," she said.
FMG shares rose 0.7% to A$19.64, about a six-week high.
