President Jefferson believed that of it wasn't already in the Constitution then that power belonged to the states instead of the Federal Government. The ability to make the purchase wasn't in the Constitution but the deal needed to be completed quickly.
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The US Constitution does not give Congress the power to buy new territory and all powers not explicitly given to Congress belong to the states. Jefferson knew he was asking the Senate to overstep their authority and ratify the purchase treaty.
Also, money had to be borrowed from Europe in order to make the purchase.
At this time, the US already had large amount of unsettled land on the east side of the Mississippi- did they really want to borrow money to get more?
One factor was that he wasn't entirely sure that he, as President, had the authority to do so. He went ahead and did it anyway since it was such an amazingly great deal and it seemed reasonable that the government in general could do so; if anyone had called him on it (no one did) then they'd have had to hash out which branch of government had that particular power.