Goldman Sachs Group Inc. mistakenly added about $1.5 million of interest costs to a Ford Motor Co. bond sale last week by using the wrong Treasury note as a benchmark for the security, according to two people with knowledge of the transaction.

As compensation, Ford was charged a lower underwriting fee for the offering of $1 billion of five-year, 2.875 percent notes from its finance unit, said one of the people, who asked not to be identified because the information is private. The automaker paid 0.1 percentage point less in fees than in two previous sales of similar-maturity debt, regulatory filings show.

Goldman Sachs, the sixth-largest underwriter of corporate bonds worldwide this year, took Ford public in 1956 and a former executive at the bank, John Thornton, sits on the carmaker's board. Such an error by the New York-based bank is rare across capital markets, said AllianceBernstein Holding LP's Ashish Shah, who wasn't involved in the sale.