What was the secret deal between spain and france?

Which one!? There was the The Third Treaty of San Ildefonso, the The Treaty of Fontainebleau...etc.

In 1801, Spain signed a secret treaty with France to return Louisiana Territory to France. Reports of the retrocession caused considerable uneasiness in the United States. Since the late 1780s, Americans had been moving westward into the Ohio and Tennessee River valleys, and these settlers were highly dependent on free access to the Mississippi River and the strategic port of New Orleans. U.S. officials feared that France, resurgent under the leadership of Napoleon Bonaparte, would soon seek to dominate the Mississippi River and access to the Gulf of Mexico.