WASHINGTON — The Treasury Department’s internal watchdog has agreed to look into why designs of a new $20 bill featuring Harriet Tubman will not be unveiled next year.

Senator Chuck Schumer of New York, the Democratic leader, last week asked the Treasury Department’s inspector general to open an investigation following Treasury Secretary Steven Mnuchin’s announcement at a May Congressional hearing that designs of the new $20 would be unveiled in 2026 instead of 2020 — the 100th anniversary of women gaining the right to vote.

Mr. Mnuchin, at the hearing, would not commit to Tubman being featured on the note, diverging from the plan and timeline set by the Obama administration and leaving the decision to a future Treasury secretary.

Treasury’s inspector general, in a letter to Mr. Schumer dated June 21, said that the review of the $20 would be included in an already-planned audit of the Bureau of Engraving and Printing’s process for designing new notes and security features. That audit will include interviews with senior officials from Treasury, the B.E.P., the Federal Reserve’s Board of Governors and the Secret Service. A formal investigation will be opened if any indication of misconduct surrounding the delay emerge during the inquiry.