Subtraction with numeric_Std doesn't work

Here is the working version. Following kaz advice.

    function detect_overflow(Tmp, A, B : in signed(7 downto 0); op : in std_logic_vector(1 downto 0)) return std_logic is
    
        variable Tmp_xtnd : signed(8 downto 0);
        variable A_xtnd : signed(8 downto 0);
        variable B_xtnd : signed(8 downto 0);
    
    begin
        Tmp_xtnd        := (Tmp(7) & Tmp);    --resize(Tmp, 9);
        A_xtnd         := (A(7) & A);            --resize(A, 9);
        B_xtnd         := (B(7) & B);             --resize(A, 9);
        
        case op is        
            when "10" =>        
                
                        Tmp_xtnd := A_xtnd + B_xtnd;
                        
                        if (Tmp_xtnd(8) /= Tmp_xtnd(7) ) then
                            return '1';
                        else
                            return '0';
                        end if;        
                
            when "11" =>
                
                    Tmp_xtnd := A_xtnd - B_xtnd;
                
                    if (Tmp_xtnd(8) /= Tmp_xtnd(7)) then
                    
                        return '1';            
                    else     
                    
                        return '0';                                            
                    end if;                    
            
            when others =>
            
                return '0';
            
            end case;                                
    
    end detect_overflow;