The Japanese tech giant is in negotiations to put just $2 billion into the US firm, the Financial Times reported.
The figure is much lower than the $16 billion discussed at the end of last year, and follows declines in global tech stocks and concern by investors over how SoftBank's $100bn Vision Fund was being spent.
While the deal has not yet been finalised - and could still fall apart - the scaling back underscores dampening enthusiasm for technology shares, which have suffered recently from a major sell-off in global equity markets.