The deal with Vietnam Rare Earths Co - which was flagged back in December - involves 100t of "product", with the pricing to reference market pricing of neodymium-praseodymium.
ASM said the deal ensures the ramp up of production at the Korean plant "as anticipated sales contracts progress this year".
The plant in South Korea came online in May last year.
ASM's ultimate plan is for the A$1.7 billion development of its resource and plant in Dubbo, New South Wales, with financing the obvious challenge.
The Rowena Smith-led ASM had $63 million at the end of March.
Shares in ASM were up 6% to $1.11 in morning trade, capitalising the company at$186 million.
