Khartoum, as part of regional efforts to end the conflict, has hosted a round of peace talks since June between arch-foes President Salva Kiir and rebel leader Riek Machar.
The factions were to sign a "preliminary" power-sharing deal on Thursday having agreed to a permanent ceasefire and the withdrawal of their forces from urban areas.
But Sudan's Foreign Minister Al-Dierdiry Ahmed told reporters the parties had made some observations that they wanted to be included in the deal.
"A new date for the preliminary signing of the deal will be announced later," Ahmed said, while insisting a final pact would be signed on July 26 in the presence of Sudanese President Omar al-Bashir.
Ahmed said the decision to postpone the signing ceremony was taken after Juba wrote to Khartoum about the draft.
"We received a written letter from the government of South Sudan, saying it wants to sign but had some observations it wanted to be included in the document," Ahmed said.
"We will look into it and then announce a new date for the preliminary signing of the document."
Rebel leader Machar's group said it was also seeking changes.
"We will not sign unless they are included in the document," Machar's representative in the negotiations, Agouk Makour, told reporters.
The warring groups had agreed on a power-sharing deal during a round of talks in Kampala on July 7 that essentially saw Machar reinstated to his previous position of vice president.
A similar deal was signed in 2015 but fell apart a year later in a deadly battle that saw Machar flee into exile.
The war in South Sudan erupted in December 2013 after Kiir accused his former deputy Machar of plotting a coup.
The conflict has already killed tens of thousands of people and forced millions from their homes.
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