It looks like either decision taken by Mush or AH at their own and does not indicate that this has been as a result of any deal with Zardari. Though Zardari wanted a formation of national government before elections.
http://www.dawn.com/2008/03/21/welcome.htm
MQM delegation meets Shujaat and formally parts ways with opposition
ISLAMABAD, March 22 (APP): Muttahida Qaumi Movement (MQM) formally parted ways with the PML-Q on Saturday by announcing withdrawal of its candidate for premiership in favour of PPP. The announcement came after a meeting of a high powered MQM delegation with PML-Q President Chaudhry Shujaat Hussain at the latter’s residence in Islamabad. Terming the decision “final” and taken in “larger interest of the people of Sindh”, Dr. Farooq Sattar, the opposition candidate for the top slot, told reporters that his party would now support the PPP candidate for the top slot, saying that this step would “help bridge the gulf between people of urban and rural Sindh.” Though Shujaat endorsed the MQM decision, tension was evident from the body language of all the PML-Q leaders present on the occasion. “We welcome the MQM decision keeping in view the negative and undemocratic trends already emerging in Sindh. Only today, a false FIR of murder has been lodged against our leader Sardar Ghous Bakhsh Khan Mehr. In this scenario, we greet the MQM decision even if they join the government in larger interest of their people. As they are our friends, we hope that being a part of treasury, they would resist any negative and undemocratic move against our leaders,” said Shujaat. However, Chaudhry Shujaat made it clear that the PML-Q would take its decision whether or not to field its candidate for the top slot. “We have convened a party meeting later in the day and final decision would be taken in the meeting,” he said. Farooq Sattar said: “Both the PML-Q and MQM have been friends and would remain so even if the PML-Q brings its own candidate and we do not caste our vote in its favour.” **For the time being, he said, the MQM was a part of opposition, and it was now up to the PPP leadership to make the choice whether or not it wanted the MQM to join the government. Shujaat also defended the MQM decision, saying that if this step is not responded fairly, the doors “are open for the MQM to return’. **Senators Mohammad Ali Durrani and Mushahid Hussain Sayed , Mian Riaz Hussain Pirzada, Chaudhry Amir Hussain, Amir Muqam, and Tariq Azeem assisted the PML-Q president during the talks while the MQM delegation included Haider Abbas Rizvi, Iqbal Mohammad Khan and Abdul Wasim. (First Posted @ 12:25 PST, Updated @ 16:45 PST)