Both sides signed a "preliminary" power-sharing agreement on 25 July, and to complete this process, negotiations will continue until a final peace agreement is signed.
A power-sharing deal will end civil war that has killed tens of thousands of people and displaced millions in the world's youngest country.
South Sudan's President Salva Kiir and Riek Machar, opposition leader and Kiir's former vice president, will attend the signing ceremony alongside many regional leaders and representatives of the global community.
Still, Ahmed said the parties disagree on the power-sharing at the level of local governments, but they are determined to solve the issue before the final signing of the document on August 5.
Mr Kiir said on Friday he was more confident that the current deal would work because it had not been "forced on us" unlike previously.
The deal also provides for 35 ministries during the transitional period.
"My government and I know the conflict in South Sudan has resulted in a financial and political burden", Kiir said.
The deal further stipulated a transitional national legislative body composed of 550 members, with 332 from the government, and 128 from the SPLM-IO.
But Sudan's Bashir said the mediators were committed to ensure that the agreement was implemented.
Concerning the boundaries and the number of states of South Sudan, it is agreed that an Independent Boundaries Commission will be established to address the issue.
Sudan's president Omar al-Bashir said on Sunday oil would be pumped from South Sudan's Wahda region to Sudan beginning September 1.
Last month the White House warned that "a narrow agreement between elites" would not solve the problems plaguing South Sudan.
A similar peace deal was signed in 2015 but fell apart a year later in a deadly battle that saw Machar flee into exile.