Although the agreement was signed in 1819, its terms were not implemented until 1821.  
By the terms of this agreement, the United States had to absorb some $5 million in lawsuits against the Spanish government, but had to pay no purchase fee.  In fact, the U.S.A. already controlled the northwestern portion of the new territory from Madison's 1810 annexation of that area.  Although the agreement was signed in 1819, its terms were not implemented until 1821.  FTP, name this agreement that led to the U.S. acquisition of the state of Florida, partially named after a future President.
By the terms of this agreement, the United States had to absorb some $5 million in lawsuits against the Spanish government, but had to pay no purchase fee.  
Adams-Onis Treaty