Collector's item, informally?

IOU

Think of what you might owe a friend after borrowing something small.

"Collector's item, informally?" plays on the idea of an "IOU" as something people might hold onto—like a debt note or a casual promise of repayment. The word "collector's item" hints at its value (even if just sentimental), while "informally" clues the abbreviation "IOU" instead of "I owe you." It’s a lighthearted twist, treating a simple debt acknowledgment as if it were a prized possession.