In the sidequest "Drugs are bad m'kay" you bring a package of drugs you found to Rififi in Bavelock. If you do so, he will buy them for 60 Pix.

The package of drugs can be found at the trapdoor leading down to the Thieves' Passage.

If you already talked to Rififi before finding the drugs, you automatically start the sidequest when you pick them up and you can simply sell them to Rififi.
If not, then you get a text saying you are not sure who would want the drugs. Find Rififi and you can still sell them to him.