Give the NPC 50 Strong Medicines (上やくそう). The 20 Flintstones will be returned. This item can't be bought from NPCs, so you'll have to get them from item drops, glittering pickups, or from the Alchemy Pot. This can also be bought from the Traveler Bazaar, but this is unavailable to Trial Players.
Strong Medicine Icon
Give the NPC 99 Single Phials (まほうの小ビン). the 50 Strong Medicines will be returned. These can easily be bought from item shops for 50G each.
Magic Phial Icon
Wait 3 real world minutes then talk to the NPC again. The Single Phials will be returned and the quest will be cleared.