Fisher said the question now for non-governmental organisations (NGOs) would be whether the funding is "new" money or is cash recycled from somewhere else.
He said: "People start looking at the figures now by looking at whether governments are meeting their pledges."
Fisher said the pledge "was really the one bright spot in what can be regarded as a lacklustre, perhaps even disappointing, G8".
Addressing the summit, Barack Obama, the US president, said the funding over three years was "in addition to the emergency humanitarian aid that we provide".
He said: "Countries in sub-Saharan Africa and elsewhere in the world that are suffering from extreme poverty have an obligation to use the assistance that's available in a way that's transparent, accountable and that builds on rule of law and other institutional reforms that will allow long-term improvement.
"There is no reason why Africa cannot be self-sufficient when it comes to food. It has sufficient arable land.
"What's lacking is the right seeds, the right irrigation but also the right kind of insitutional methods that ensure that a farmer is going to be able to grow crops, take them to market, get a fair price."
Speaking to Al Jazeera, Jacques Diouf, the head of the UN Food and Agriculture Organisation, said it was the "first time in two decades the focus is on how to help the hungry people".
Diouf said: "The most important thing is that now we focus on production instead of only focusing on food aid."
"The whole question of when will this be mobilised ... how will it be co-ordinated and under which conditions will we use this money - this will be discussed after the G8 summit."
Silvio Berlusconi, the Italian prime minister, who hosted the summit of the G8 group, which comprises US, Canada, UK, France, Italy, Germany, Russia and Japan, said the agreement was a "tangible and significant result".
According to the United Nations, the number of malnourished people in the world now exceeds one billion.
Stephen Chan, a professor of African studies at the School of Oriental and African Studies in London, told Al Jazeera that it was important for poor countries to have self sufficiency.
"Getting food security ... particularly in a productive way so that people become self-sufficient by way of growing food themselves, I think that's extremely important," he said.
"The whole question of very cheap food exports, food dumping as it were from Western countries ... can be a very real problem because that provides a disincentive for local farmers to be able to provide their own agricultural products."
Chan said open markets become meaningful "when there's competition that goes both ways, when African states for instance are also able to compete against Western producers. And we're a long way from that right now".
Charity workers previously expressed disappointment at what they saw as broken promises from leaders, after Italy announced a 56 per cent cut in its aid budget this year.