from you message it seems you are needing the money to pay for rental/whatever else in Singapore when you move..
if you ask me, the safest and probably most straight forward way is to just carry 40k HKD over in cash, go over to the Arcade (Raffles Place) and check all the money changers there (there are probably 30 of them)... you won't be surprised you can actually get better rate than bank or even transfer wise because sometimes these guys are short of HKD and they are willing to pay half a cent more to get stocks..