The commissioner's decision was made after another charity that had bid unsuccessfully made a request under the Freedom of Information (Scotland) Act.
Barnardo's Scotland and Glasgow Simon Community complained that the commissioner did not contact them before concluding that releasing the proposal would not prejudice their commercial interests or trade secrets, and that the release was in the public interest.
They called on the authorities to clarify the tendering procedures in the light of the ruling.
Benjamin James, a partner at law firm Bircham Dyson Bell, said Scottish freedom of information rules for commercial interests were different from those in England. "However, the principles relating to public interest are similar," he said.