Giving a Pocket Knife As a Present.......

I once read that the recipient should then give the presenter a coin, but I have forgotten the reason. Can anyone enlighten me?

Thanks

Q

I looked it up a=on google and found this.

http://www.royalinsight.gov.uk/200012/out_and_about/sheffield/sheffield.html

I’ve seen this in Germany as well as here. Usually the coin is a penny. Last knife store I went to asked me for one at the end of the transaction just for luck.