In that case, you're paying for the time your employees could have spent elsewhere - and you're likely not paying more if your employees make more sandwiches faster. Also, every sandwich is unique, and takes physical effort and resources to make - if sandwiches could be copied as easily as software, chances are you would fire all but one employee and pay that one as little as you legally could, and still charge the same price, but would that really be fair?
The problem with the economy around digital media is justifying what, exactly, the consumer is meant to be paying for.
The contractual debt you incurred when the work happened. Or, in terms of expected value received from paying, you are paying to avoid the legal consequences of not paying that debt.
Temporality is a nonsense argument.
Your sandwich shop worker served you and 80 other customers last Tuesday. I'm the boss, and it's time for me to cut the checks. Wait—what am I paying for here? Today's Friday, and all those sandwiches are already out the door.