These outstanding FOI requests cover about 1.3 million receipts for MPs' spending on second homes, travel and office costs.
The High Court ruled last year that details of claims made under the £24,000-a-year second homes allowance, including receipts, should be published in the interests of transparency.
But Ms Harman said proposed changes to how expenses would be audited in future, including independent scrutiny, may change the "legal position" regarding disclosure.
"There is both the legal decision, but there is (also) the public policy decision of whether or not, having published receipts once, it is sensible to go back on publishing them and not publish them in the same way in the future," she added.
The Conservatives said the issue of full disclosure under FOI was "still hanging".
"My view is that we should still be subject to Freedom of Information," said shadow commons leader Alan Duncan.
MPs should observe the "highest standards" of transparency, he said, but there should be some safeguards in place.
"We don't just want, by making everything available, to allow ourselves to be subject to open season of malicious and vexatious attacks," he added.
The Lib Dems said Parliament should have responded to public pressure for more openness many years before.
"The tragedy is that those who didn't want to disclose anything have had to end up disclosing everything because they couldn't see the way the wind was blowing," said Lib Dem commons spokesman David Heath.
MPs approved without a vote moves to increase the scrutiny of their expenses and the rules under which they are declared.
The proposals will give the National Audit Office new powers to scrutinise expenses while claims would be broken down into 26 categories rather than the current 14.
Information about telephone and energy bills, as well as mortgage and rental payments on second homes, will be published for the first time.
Freedom of information campaigners have insisted that all receipts for individual claims must be published to maintain public confidence in Parliament and to uphold standards.
Daily Politics: Two MPs on whether details of all their expenses should be published
"Some MPs are clearly desperate to prevent the release of past expenses claims which are likely to have exceeded what could reasonably be justified to the public," said Maurice Frankel, from the Campaign for Freedom of Information.
The Scottish Parliament already publishes details of its members' claims, including receipts, on the internet.
But UK ministers have expressed concerns about the cost of doing this.
MP's expenses have been the subject of controversy since it emerged last year that Conservative MP Derek Conway had paid his son, a student, more than £40,000 to act as a researcher when an inquiry found he had done no work.
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