On this page of the Disco Elysium game guide, you will find an accurate walkthrough description of the Return Lena's pin side task.
The task Lena's Return Pin in Disco Elysium appears at the moment when, looking for cash, you asked for help from a woman sitting in a wheelchair - then you will receive the item Lena's pin.
The pin can be pawned at Roy's Pawnshop and thus receive a small sum of money. The task will end when you buy back the pin from Roy and return it to Lena . Alternatively, you can give Lena money by doing the side task Pay Lena back for pin.