Hello again crl,
I am not sure what I can elaborate on, other than you are not allowed to deduct an expense which you actually did not incur.
In other words, just because the invoice was not paid in time to take advantage of the discount that was being offered does not mean that you actually incurred an additional expense. It just means that you you were not able to take advantage of a discount that was offered to you. It's almost the same thing as booking a higher price that you paid for paper because you happened to purchase it one week after a sale had ended. You can't book the difference as an expense.
Let's say that the paper you purchased cost $500, and they were offering you a $20 discount if you paid the invoice in 10 days. If you had paid the invoice in 10 days, then you would have sent the vendor a check for $480. You would have then booked the $500 as an expense for the cost of the paper, and then booked the $20 as income from the purchase discount. Purchase discounts are treated as income accounts for accounting
purposes. So your total out of pocket expense would have ended up netting out at $480.
When you missed the deadline
date to take the discount you simply still have the $500 cost of the paper, but you have no income to book from the purchase discount, because you were not able to take the discount and you paid the full $500. But now if you come along and book the missed discount as an additional expense, in effect you are saying that the paper cost you $520, which it did not.
Not taking a purchase discount does not increase your cost of the product. You still paid no more for the product than if the vendor had not offered the discount in the first place. The price of the paper was never $520, and you did not pay $520. You only paid $500, and that is the only expense you can book.
I hope this helps explain it, but let me know if you have more questions.