Final answer:
The nurse should dispose of the unused medication in the proper waste container.
Step-by-step explanation:
The nurse should dispose of the unused medication in the proper waste container. It is important to follow proper medication disposal procedures to prevent misuse or accidental ingestion. Requesting another nurse to witness wasting the medication or storing it in the medication cart is not necessary in this situation. Returning the unused medication to the dispensing system is also not recommended as it could lead to medication errors or potential contamination.