The US State Department sent its agreement for Ms Rehman to the Pakistan Embassy in Washington earlier this week and the mission intended to send it to Islamabad today, the Dawn
newspaper quoted diplomatic sources as saying.
"Once the documents arrive there, it will take a day or two to complete the formalities," a diplomatic source told the daily.
The source confirmed that Ms Rehman, a senior leader of the ruling Pakistan People's Party, would arrive in Washington next Monday.
Diplomatic courtesy requires that before a country sends a new envoy, it must first ascertain whether the proposed appointee is acceptable to the host country.
The receiving state conveys its acquiescence by granting its agreement to the appointment.
The US State Department has already said that it knows Ms Rehman and is looking forward to working together with her.
The problem revolves around efforts to rebuild "a strong, cooperative relationship" between Pakistan and the US, State Department spokesman Mark Toner said while welcoming
Ms Rehman will take over at a time when US-Pakistan relations are at their lowest.
The US Congress has slapped new restrictions on aid to Pakistan and President Barack Obama has signed a bill containing these restrictions into law.
However, the US Secretaries of State and Defence can still maintain the flow of aid to Pakistan by certifying that Islamabad is making efforts to curb the use of IEDs.
Diplomatic observers in Washington said recent developments would leave a "very narrow space" for Ms Rehman to operate.
They argued that her success or failure would depend mostly on relations between the US and Pakistani defence establishments.
Mr Haqqani was forced to resign after he was linked to the alleged memo that sought US help to stave off a feared military coup in Pakistan after the killing of Osama bin Laden