Two distinctions matter:
1. If the merchandise is absent or not ready for delivery, commentaries do not require the buyer to pay regardless of the seller's ability to hand it over.
2. If the parties validly stipulated a deferred price, immediate cash payment is not due merely because a sale exists.
“Every buyer must always pay before any delivery” is an overgeneralization. The text's paradigm is a present commodity for an immediately due price.