New York-based Somerset Partners LLC paid $509 million for 450 Park Ave., the most ever paid for an American office tower on a per-square-foot basis.

The closely held private equity firm paid about $1,566 a square foot for the 325,000-square-foot tower when the transaction was completed on Friday, a Somerset principal, Keith Rubenstein, said. The seller was Taconic Investment Partners LLC, along with New York State Common Retirement Fund.

“Never for a second” was the deal delayed or in danger because of the tightening of credit, Mr. Rubenstein said. “From the time we entered into contract our plan was always to assume the existing loan, and use our equity for the balance, and that’s exactly what we did.”

New York office brokers and investors have been looking for signs that pricing would remain at record levels since August, when the market for securitized commercial mortgages slowed. Setbacks in residential lending triggered a retreat from commercial loans for more than 80% of property value, removing highly leveraged potential buyers from the market.

The building sits at the intersection of New York’s most expensive office street, Park Avenue, and one of the most expensive shopping districts in the city, East 57th Street.

Its tenants include Tudor Investment Corp., the hedge-fund firm run by Paul Tudor Jones, and the law firm of Cleary Gottlieb Steen & Hamilton LLP, which maintains conference rooms there.

New York benefits from the exchange rate between the dollar and the pound and euro, Mr. Rubenstein said, whose company invests on behalf of European investors.

“If you look at London, the pound is two to one over the dollar,” he said. “If you have pounds converting to dollars, prices for trophy assets in New York are relatively half the price of what they would be in London. If you’re a long-term player, and you have European currency, it makes very good sense to buy in New York.”

The Park Avenue transaction exceeded the previous record of $1,476 a square foot set in August paid by Risanamento SpA, Italy’s third-largest real estate company, for the offices at 660 Madison Ave., a 23-story tower at Madison and East 61st Street. That building houses the flagship store of Barneys New York, the luxury clothier.

The agreement to sell 450 Park was made in late June, Mr. Rubenstein said. Somerset, led by Mr. Rubenstein and Marshall Allan, is assuming a $175 million mortgage on the property with a 5.6% interest rate.

He said he expected the property to yield between 3 and 3.5% in the first year, and for income to rise, since about half the building’s leases expire over the next few years.

In a July interview, he said the rents could double from the current $65 a square foot, based on prevailing rents in Park Avenue office properties.

The sale was negotiated by a broker with New York-based Eastdil Secured LLC, Douglas Harmon, who also brokered the 660 Madison transaction.