New York City mayor Michael R. Bloomberg has made a low-interest loan of $15 million to the National September 11 Memorial and Museum to help cover its expenses until the museum opens to the public at the former World Trade Center site, the New York Times reports.

To ensure that the transaction qualifies as a loan rather than a gift, Bloomberg, who serves as chair of the 9/11 Memorial and Museum, is charging the lowest possible interest rate — less than 0.3 percent — through a corporate entity of which he is the sole shareholder. The loan will help address the occasional cash-flow problems experienced by the foundation set up to build and operate the museum, which originally was scheduled to open in September 2012. The cost of building the 100,000-square-foot underground museum has grown to more than $700 million, and while state and federal governments and the Port Authority for New York and New Jersey have provided $425 million toward construction and private donors have pledged more than $450 million — including $15 million from Bloomberg — many of the largest pledges will pay out over years, contributing to delays that in turn drive up the cost of the project.

According to the Times, the foundation had sought a bridge loan from a bank, but most lenders were reluctant to make a loan against pledges. Although JPMorgan Chase did offer to provide a loan, the interest rate and fees would have been four percentage points higher than the loan from Bloomberg, foundation president Joseph Daniels told the Times.

At its board meeting last week, the foundation also voted to charge an entry fee — expected to be between $20 and $25 — to the museum, which is now scheduled to open next April. The above-ground memorial itself remains free to the public, although the foundation recently implemented a $2 service fee for online reservations.