The Al Khalaf Group is a big player in livestock transportation, and has farming interests in Hawkes Bay with two properties totalling 2150ha.
After touring Saudi Arabia and other Gulf states last month, Prime Minister John Key told media there was no issue of a disgruntled Saudi investor opposing a deal.
Labour's trade spokesman David Parker said if the statements attributed to Mr Assaf were true, the farm investment was disgraceful.
"You shouldn't buy free trade agreements. You shouldn't have to do special favours for powerful people," Mr Parker told the Weekend Herald.
"I think [the Government] is undermining the business ethic in New Zealand and the international reputation we have."
In a statement, Mr McCully said it was estimated the Al Khalaf Group had contributed about $80 million in land, labour and finance towards the project.
"This agri-hub is a mutually beneficial project with 30 New Zealand businesses attached to it. The companies each stand to gain ...
"While furthering our FTA ambitions with the Gulf states was a key consideration in the establishment of the hub, the investment, and our wider food security co-operation partnership with the Gulf states, would not have gone ahead if it did not provide a sound opportunity for New Zealand companies."