check Enum value of RetailStoreTable from RetailStatementTable Form

i created new column in RetailStoreTable type Enum (acceptnegativevalue) and type of this enum is (NoYes) as you see here in Retail stores

now i want check this value when i press post statement  in open statement form 

so i edit main code of post statement for check if total Difference is negative or positive to this code

void clicked()
{
    if(Sale_M_totalDifferenceStore.realValue()>=0)
    {
    super();
    statementLine_ds.executeQuery();
    element.close();
    }
    else
    {
        info ("in this case can't accept negative value you must change case");
    }
}

so i need to check (acceptnegativevalue) first befor start my previous code

how can i add this into my code?