Take PayPal or Wise. User A sends $100 USD to User B, and the payment network takes a $1 fee. User A now has -$100 USD and User B has $99 USD. If you add -100 + 99 it might seem like money disappears, but that remaining $1 of value has gone to the network as a fee for services rendered, and it was something that both parties were willing to pay.

Even here, both users are short USD, but it is not a "negative sum" system as the users gain some positive value by having the payment processor transfer and secure their funds. If the only metric is how much USD a buyer has left, every visit to the bakery would be a negative sum game.

In the above example, PayPal and Wise are User C. In a permissionless blockchain, User A and B can both also opt to be validators and receive these protocol rewards, so there may not even be a User C that extracts rent on each transaction.