The deal would value Sprint shares near its market price of $6.50, sources told CNBC.
The terms of the deal would allow Deutsche Telekom, which currently owns nearly two-thirds of T-Mobile, to consolidate the combined company’s earnings. SoftBank owns 85 percent of Sprint.
Shares initially plunged as much as 10 percent after Bloomberg reported the deal would value Sprint at $24 billion, or about $6.10 per share. That price undervalues Sprint by about 40 cents per share. Sprint pared some losses and was last trading down 8.3 percent.
The deal could be announced as soon as Sunday, though sources warned the deal was not final and may still fall through. The companies came close to a merger agreement in November, before SoftBank CEO Masayoshi Son pulled out after weeks of talks.
Shares of Sprint are up more than 10 percent year-to-date.
–Thomson Reuters contributed to this report.