Someone else gave me the suggestion of giving them a fake sample instead! A vial of dog's blood, or something similar.

[That could work? Maybe? It also sounds like a terrible idea, though.] If they can't tell the difference, then they mustn't be a very good at what they're trying to do, right?

[Or, they could go find the requestor and realize it's a 10,000 year old witch who will curse them both into becoming frogs. Either way.]