The pair are struggling over these issues in talks, but Softbank, which controls Sprint, is not planning to withdraw from the deal talks as soon as Tuesday as some reports suggest, these sources say.
Nikkei earlier reported the point of contention was over T-Mobile owner Deutsche Telekom's request to have control over the combined company. SoftBank reportedly decided it "would not give up control."
"SoftBank Group plans to break off negotiations on a merger between subsidiary Sprint and T-Mobile US amid a failure to agree on ownership of the combined entity," the Japanese financial news agency said.
Reuters and Dow Jones both confirmed that SoftBank was hesitant to give up control of Sprint.
Sprint shares dropped as much as 13 percent, while T-Mobile stock declined as much as 5 percent after the earlier reports. The shares of both companies pared some of their losses.