TOKYO, June 10 -- The most aggressive financier of the artificial intelligence boom asked the world's banks a simple question: how much will you lend against the most valuable startup ever created? The answer, for now, is a pause.

SoftBank Group's talks to raise at least $6 billion through a margin loan backed by its OpenAI stake have stalled, Bloomberg reported on Wednesday, and SoftBank's Tokyo-listed shares fell on the news. Some prospective creditors balked at the problem of valuing OpenAI, a private company, as collateral, and at reports that the startup has missed some internal sales and user milestones in recent quarters. The loan was structured with a two-year term and a one-year extension option.

The stall is the second downgra...