You misunderstood. For those in purgatory, their sins are forgiven. The Blood of Jesus is applied to them. But the temporal punishment for these sins that is left is what is being repaid there.
Then you don't understand salvation. If Jesus paid the penalty for our sins on the cross, how come there is some penalty left over for us to pay. If there is, then His blood was not sufficient to wash away our sins. If we take on His righteousness because of the cross, how come we still have to pay a penalty for our unrighteousness that He said he took on the cross? Purgatory cannot be argued from the scriptures without a great deal of manipulation.