I think this custom goes back to the days of duels and the rules of honour/chivalry and such.

To offer someone a weapon was an invitation to single combat (or perhaps the full entourage, including seconds, etc.).

However, if money was exchanged, in the slightest amount, the exchange of weapons was a sale, a transaction, and not a challenge.

It was wise in those days to be clear about such things. Perhaps it still is. :-)