@burningbloom the medicine was shown on the previous chapters but it was never used, I believe it is to control Sung Woo in case he doesn't agree to the proposition.
@heisenbergeth@burningbloom I understood that as a medecine meant to erase people's memory (when someone who shouldn't know learns about non-humans for ex, or in this case if he had reacted negatively to Leo's telling him and asking him to be his 'gardener' or if he had choose that day to forget), maybe?
As for the contract I thought they possibly mean a magical contract maybe to bond Sung Woo with Leo for mutual protection or something?