Put the cucumber and scallion in a small mixing bowl and add the dressing. Mix well and let sit for 10 minutes before serving.
In a small saucepan, combine the sea salt, fish sauce, palm sugar, and granulated sugar and cook over low heat, stirring, just until the sea salt and sugar are dissolved. Remove from the heat and transfer to a mixing bowl to cool. When cooled completely, stir in the lime juice.