How did President Jefferson go against one of his principles in order to buy land?

This question refers to Jefferson's decision to go through with the Louisiana Purchase from France in 1803. Jefferson had always favored a strict interpretation of the Constitution--he thought that the government should not exercise powers that were not expressly described in the Constitution. On these grounds he had opposed the chartering of a national bank, among other acts of government under Federalist leadership. When Jefferson was presented with the chance to purchase the vast territory of Louisiana at about three cents an acre, he knew the offer was too good to pass up. Yet the Constitution did not give the President the power to purchase territory. Despite his misgivings, Jefferson signed off on the deal, which was later approved by Congress (with some opposition from the opposing Federalist faction). In so doing, he not only secured American control of the Mississippi River, but doubled the size of American territory.