«My Brother, the Druggist» by Marvin Kaye

While on vacation in DC, Marty must outwit kidnappers at a magicians' convention.
Marty Gold deserves a vacation. For years he has toiled behind the pharmacy counter at Spector's, a Manhattan institution whose classic soda fountain makes it a magnet for every overstuffed rear end on the West Side. Among his most devoted customers is Mase O'Dwyer, a chunky young magician who treats Marty as a captive audience for hour upon hour of poorly executed magic tricks. When Marty finally saves up enough money for a jaunt down to Washington, DC, Mase insists on tagging along to attend a magic convention. But as soon as he arrives, the hapless magician finally manages to make one trick work: He disappears.
Mase has been kidnapped, and as much as he dislikes the kid, Marty feels obligated to rescue him. It will take magic to save the portly illusionist, but the druggist has a few of his own tricks up his sleeves.