“Now where remission of these is, there is no more offering for sin.” (Hebrews 10:18, KJV 1900)
Once something is paid it is paid. Years ago, I worked for a company in Zurich Switzerland as Manager of Engineering and Service in their U.S. field service office. One of the companies to which we sold a printing press purchased a large order of repair parts with the machine. The parts were sold through the U.S. office and I forwarded them the invoice. They were a bit late on payment, so I sent then a reminder. Right after mailing the reminder, we received the payment.
However, about three weeks later we received another payment of the same amount. They paid the reminder statement that I had sent them. I called the Zurich office to see what to do and they told me to send a credit memo for the over payment so the customer could use this for further purchases.
About a month after I sent out the credit memo, the customer sent us another check for the same amount. Somehow, they paid the credit memo to us rather than taking it as a credit. At this stage they paid us three times for the same goods received. (Rather than paying around $30,000 for $30,000 worth of parts, they paid us $90,000.)
Well, the managing director of our company was coming from the Swiss office to visit this customer and decided to handle the issue himself. He refunded the company all the money they sent in the overpayments. The point is that once something is paid in full, there is no more debt to be paid.
The author of Hebrews is making this point. Under the Old Covenant, the sacrifices under that covenant were never sufficient to pay the penalty of sin. They were a foreshadowing of that which would pay the complete debt of sin, the sacrificial death of Jesus on the cross. In the New Covenant, as recorded by the prophet Jeremiah and reiterated by the author of Hebrews, Jesus dealt with the problem of sin once and for all. “For I will forgive their iniquity, And I will remember their sin no more.” (Jeremiah 31:34, KJV)
Since Christ initiated this New Covenant by His shed blood, He has brought about the fulfillment of the covenant promise. God will forgive believers’ sin and never again remember their sins against them. Thus, the sacrificial offering of Jesus, unlike the offerings under the Old Covenant, never needs repeating. Once the debt of sin is paid, there are not further payments needed.