On April 11, 1803, Talleyrand asked Robert Livingston how much the United States was prepared to pay for Louisiana. Livingston was confused, as his instructions only covered the purchase of New Orleans and the immediate area, not the entire Louisiana territory. James Monroe agreed with Livingston that Napoleon might withdraw this offer at any time. To wait for approval from President Jefferson might take months, so Livingston and Monroe decided to open negotiations immediately. By April 30, they closed a deal for the purchase of the entire 828,000 square mile Louisiana territory for 60 million Francs (approximately $15 million). Part of this sum was used to forgive debts owed by France to the United States. The payment was made in United States bonds, which Napoleon sold at face value to the Dutch firm of Hope and Company, and the British banking house of Baring, at a discount of 87 1/2 per each $100 unit. As a result, Napoleon received only $8,831,250 in cash for Louisiana. Dutiful banker Alexander Baring conferred with Marbois in Paris, shuttled to the United States to pick up the bonds, took them to Britain, and returned to France with the money - and Napoleon used these funds to wage war against Baring's own country!

When news of the purchase reached the United States, President Jefferson was surprised. He had authorized the expenditure of $10 million for a port city, and instead received treaties committing the government to spend $15 million on a land package which would double the size of the country. Jefferson's political opponents in the Federalist Party argued that the Louisiana purchase was a worthless desert, and that the Constitution did not provide for the acquisition of new land or negotiating treaties without the consent of the Senate. What really worried the opposition was the new states which would inevitably be carved from the Louisiana territory, strengthening Western and Southern interests in Congress, and further reducing the influence of New England Federalists in national affairs. President Jefferson was an enthusiastic supporter of westward expansion, and held firm in his support for the treaty. Despite Federalist objections, the U.S. Senate ratified the Louisiana treaty in the autumn of 1803.
 

A transfer ceremony was held in New Orleans on November 29, 1803. Since the Louisiana territory had never officially been turned over to the French, the Spanish took down their flag, and the French raised theirs. The following day, General James Wilkinson accepted possession of New Orleans for the United States. A similar ceremony was held in St. Louis on March 9, 1804, when a French tricolor was raised near the river, replacing the Spanish national flag. The following day, Captain Amos Stoddard of the First U.S. Artillery marched his troops into town and ran the stars and stripes up the fort's flagpole. The Louisiana territory was officially transferred to the United States government, represented by Meriwether Lewis.