Correct me if I’m wrong, but it sounds like you’re referring to the passage in 1 Corinthians 5 where St. Peter says “you are to hand this man over to Satan for the destruction of the flesh, so that his spirit may be saved in the day of the Lord.” Of note, he said this in the context of someone committing fornication and scandalizing the flock. If you read the entire chapter together, he is saying it is better for this particular man to be cast out “to Satan” in the world (which at the time were often associated together since Satan is evil and the world is corrupt) than to be participating in the Church because of the scandal he is causing others. I think you’d agree with me that Satan can still tempt us, though he has been condemned to Hell. St. Peter I think is hoping the shock of being put out of the Church will wake this guy up so that, even if his flesh and body suffers, perhaps his soul might be saved. If it was simply for correction, why is Peter then so concerned with “saving his soul?”
Jesus also speaks often of the “eternal fire.” For instance, paraphrasing Mark 9:44 “…It is better for thee to enter lame into life everlasting , than having two feet, to be cast into the hell of unquenchable fire.” Also Matthew 25:41, where Jesus is speaking of the final judgement, where he separates people into the right and into the left: “Then he shall say to them also that be on his left hand: Depart from me, you cursed, into everlasting fire which was prepared for the devil and his angels.” And then in verse 46: “These shall go into everlasting punishment: but the just, into life everlasting.” So you can see there are two different options for us, at the end.
I think you would agree with me that Hell certainly is everlasting, and God would never tell us to depart from Him unless we had chosen not to be with Him, which is the true Unforgiveable Sin. To reject God, and the movement of the Holy Spirit within us. To willingly choose to turn aside from Him, just as Satan did. The angels got the choice, and so do we.
As for paying the last penny, you may be looking at the concept of Purgatory. To paraphrase Matthew 12:32 Jesus says "…whoever speaks against the Holy Spirit will not be forgiven, either in this age or “in the age to come.” So, though we’ve found there is indeed an “eternal fire,” there is also forgiveness that comes after death. Catholics define this as a state of purification: Purgatory. Those souls who maybe did wrong things most or all of their life, but at the moment of death, the final choice, they chose to be friends with God, to be with Him forever. Beautiful!
However, God is perfect and desires us to be perfect as well, so to allow these particular souls to make up for some of the unworthy things they did on Earth, He will grant them a time of purification, to obtain that purity which He originally created us for, and so that we might truly enjoy heaven. Imagine entering a fancy party in a dress that you spilt coffee on! I would want to fit in, so I would spend time cleaning the coffee off before being able to be comfortable at the party. God understands us better than anyone