procedure TForm1.Button1Click(Sender: TObject);
begin
 if (Sender as Tbutton).Tag=2 then
  (Sender as Tbutton).Tag:=123;
begin
первая часть кода
end;
while (Sender as Tbutton).Tag<>123 do
begin
application.processmessage;
sleep(1000);
end;
end;