Java Final Variable Initialization in static block

Question

Assume the case:

public class Executor {

    public static class Properties {
        public final static String SOME_PROPERTY;

        static {                
            java.util.Properties properties = PropertiesReader.readProperties();

            SOME_PROPERTY = properties.getProperty("some.property");
        }
    }
}

Here all is fine and code compiles. But when we change it this way:

public class Executor {

    public static class Properties {
        public final static String SOME_PROPERTY;

        static {                
            java.util.Properties properties = PropertiesReader.readProperties();
            // The changes go here
            Properties.SOME_PROPERTY = properties.getProperty("some.property");
        }
    }
}

It's for some reason fails, with compilation error that

"final field could not be assigned"

Could you please help by explaining the difference between first and second sample?