Why did Jefferson, who believed in a strict interpretation of the Constitution, decide that the President had the power to buy land when that was not mentioned in the Constitution? If this is your question, here is the correct

In the end, Jefferson decided that he did have the authority to buy Louisiana. The Constitution, he reasoned, allowed the President to make treaties, and buying the Louisiana territory was part of a treaty. Federalists opposed the purchase as unconstitutional and feared it would weaken the other states. But the Democratic Republicans supported it, and the Senate approved the treaty. The Louisiana Purchase went into effect. In 1803, the United States took control of the vast lands west of the Mississippi. With one stroke, the size of the nation had almost doubled.