An indulgence is what it is, and has no necessary connection to purgatory. Should I so wish, I could go about collecting indulgences soley for my own spiritual benefit, without any heed towards te Holy Souls.
What an indulgence is, specifically, is a remission of all (plenary) or a set amount (partial) of the canonical penance due a sin, in view of the excess merits of Christ and the Saints. For example, say a saint in their lifetime performed sufficent penance for their own sins 100 times over, plus they also suffered martyrdom. They could communicate this excess of penitential acts to other members of the body of Christ by applying them to the other person. Or it could be communicated for them by the Pope or Bishops in view of the power of the keys (St. Matthew 16.19, 18.18) to bind and loose all things.
Here is where what St. Cyprian is doing comes into view. He is permitting the Martyrs to promise a share of their suffering and future prayers to the lapsed to allow the lapsed to complete their penance. The only difference here from an indulgence as practiced today is that the Martyr is not yet dead and so is granting his merits in person, and the forgiveness of penance is not for a set period of time. If we truly believe in the communion of saints, the first difference should be irrelevant. The second is also irrelevant, because St. Cyprian could have set the letters of peace as having a set term if he had so chosen.
I think what is probably confusing you is the notion that some had that you could collect indulgences of 80 days or 3 years or 15 years and that these represented a fixed reduction of time in purgatory. That is certainly not the case in reality because it is creating an exact identity between canonical penance and the temporal satisfaction due to God. The penance represents the Church's estimation of what the sinner owes God and the body of Christ for the disorders he introduced by his sins. The temporal punishment is God's reckoning in the scales of justice.
Also an indulgence applied towards purgatory is done so in the mode of a suffrage or prayer, and not after the manner of an absolution from temporal punishment. The indulgence pays what is necessary for the soul to come towards bliss - we don't presume to judge the sincerity of the dead in finalizing their penance as we might on earth in reducing penance for a sincere confession.
" The indulgence pays what is necessary for the soul to come towards bliss "
Orthodoxy doesn't teach "payment" by individuals for coming to theosis, HC.