The explanation I heard…
I heard that the reason you were supposed to give a coin when you received a knife as a gift was that this indicated that you “bought” the knife of your own free will. Otherwise, if you cut yourself with the knife you might be tempted to blame the giver.
BTW-- It IS a Jewish custom to give wine to a Mohel (“Moil”) but only AFTER he’s done his work… 
The information I glean
from P-Net!
Jim
Well, maybe a sip to stead his hands
would be alright.
Ohhhhh
-here I always thought those Jews were just being stingy.