the popular version is that the US initiated the talks for selling the port. you can POD away having a president who is proactive, and have the US taking the attitude of just looking to have guarantees of open shipping instead of looking to take ownership of the port. While Manifest Destiny in hindsight looks like a pre-ordained thing, that really isn't the case. France could end up holding a territory it no longer really wants, in which case Spain continues to govern the region for France, as in OTL, until France breaks with Spain and Spain simply reclaims it as theirs.