The Upper House Budget Committee on March 20 formally approved the summoning of Sagawa on March 27 as a sworn witness, which would make him subject to perjury charges.
The prime minister's remarks were made Tuesday to Natsuo Yamaguchi, the head of the ruling Liberal Democratic Party's (LDP) coalition partner Komeito party.
Aso, who doubles as deputy prime minister, has also resisted calls from the opposition for him to step down over the revisions made to the documents. A Mainichi Shimbun poll similarly found 68 percent believe Abe bears responsibility.
Abe's support ratings have dropped sharply since the finance ministry said it had altered records relating to a discounted sale of state-owned land to school operator Moritomo Gakuen, which had ties to Mr Abe's wife, Akie.
It later emerged that Moritomo Gakuen had paid about a sixth of the market price.
Ministry officials said the land deal documents were altered from late February to April previous year at the instruction of the financial bureau, the ministry department in charge of state property transactions, mostly at its regional unit in Osaka.
Ministry after Akie Abe was quoted by Moritomo chief Yasunori Kagoike as saying, "Please go ahead with" the project in her talks with a ministry official in April 2014.
Aso has blamed the alterations on "some staff members" at the ministry.
Ministry officials deleted all mentions of Akie Abe from the papers to hide her apparent influence on the negotiations, according to Koike.
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Japan's Finance Ministry admitted last week that 14 documents about the Kindergarten land deal were found to be tampered with and the land was sold with the help of the Prime Minister's wife at lower rates.
Opposing sworn testimony by Sagawa also risks backlash against the LDP and its leadership.
"You still can't tell what damage (the Moritomo scandal) will have on Abe as far as the September presidential election is concerned", Matsumoto added.
The prime minister said Tuesday that "the government will thoroughly investigate the situation behind [the alterations] and comply with questioning" in order to "regain the public's trust".
After the scandal first came to light in February a year ago, Akie quickly severed her affiliation with the school and its operator.
Support, however, eventually rebounded due to the unpopularity of the fragmented opposition camp, Matsumoto pointed out.
"Sagawa was probably acting to protect someone", said Kiyomi Tsujimoto, Diet affairs chief for the center-left Constitutional Democratic Party, suggesting Abe's statement spurred the deletion of references to Akie Abe.
If Abe is to be replaced, it will require an internal power struggle within the LDP, and the opposition camp will play no part, Matsumoto said.
While leaders from other industrial nations in Europe and North America have seen relations with Russian Federation sour, Abe has maintained cordial ties with Putin as he seeks Russia's support in dealing with North Korea and the return of islands occupied by Russian Federation since the end of World War Two.