There is a widespread folk superstition that a gifted blade cuts the friendship, and an equally widespread remedy: the recipient gives a small coin back, usually a penny, which turns the gift into a symbolic purchase. Whether you believe it or not, including a coin is a nice touch that costs nothing.
The belief appears across many cultures and the remedy is consistent: the recipient hands the giver a small coin, which makes the transaction a sale rather than a gift. No blade was given, so no friendship is cut.
It is folklore rather than fact, and it is enjoyable folklore, which is why it has survived so well among knife people.
Tape a penny to the box or tuck it inside with a note explaining that they owe you one cent. It turns a small superstition into part of the gift, and most recipients find it charming whether or not they take it seriously.
See the penny question.
Some recipients hold the belief sincerely, particularly in wedding and cross-cultural gifting. Including a coin costs nothing and removes any awkwardness entirely.