Citing recent trips to the US and UK, he said Western donors, where much of the funding is expected to come from, were sceptical and demanding more reforms.
"This is the best chance Zimbabweans have had for peace and prosperity in decades"
South African Nobel laureate and chairman of The Elders
"There are some Western colleagues who still have some bones to chew with us, and we understand," Biti said. "We welcome African institutions that want to help us."
Major Western donors have insisted that Mugabe make more concrete steps toward reform, by releasing political prisoners and cracking down on violence on the nation's farms.
Zimbabwe officials are seeking to raise about $1bn from the African continent.
Neighbouring countries have already pledged $400m to help Zimbabwe's economic recovery, and on Wednesday the International Monetary Fund (IMF) announced that it would resume technical assistance to Harare.
The earlier credit lines were extended by the Common Market for Eastern and Southern Africa, the regional trade bloc, as well as South Africa and Botswana.
The IMF has offered targeted help in key areas including tax policy and administration, payments systems, banking supervision and central banking governance.
Following the IMF announcement members of a group of leading international figures known as The Elders, wrote to donors and the European Commission urging them to respond more swiftly on aid.
Kofi Annan, the former UN secretary general, and Desmond Tutu, the South African Nobel laureate, said speedier action was necessary to help stabilise Zimbabwe.
"The inclusive government needs more support to ensure that it can initiate the urgent stabilisation and early recovery programmes that the people so desperately need," Tutu, chairman of The Elders, said in a statement.
"Now is not the time for donors to take a 'wait and see' approach. This is the best chance Zimbabweans have had for peace and prosperity in decades."