This would probably be better posted in another section like “Sprituality” or “Apologetics”. I am not the best person to respond but I’ll take a stab at it, and others can correct me.
An Indulgence does not involve the church directly as a third party. The norms are set forth in advance by the church and the believer goes about the business of satisfying these guidelines on his own. There are no records kept, no ‘tickets’ or receipts, nothing at all like that.
Without getting too specific about all this I can say that the indulgence is intended to encourage certain bahavor or practice. One makes a good confession, abandons all attachment to sin and does a series of specific prayers or attends a Mass or Masses. Perhaps someone here can post a typical indulgence for us to see how it is worded these days.
The result of this is the remission of a certain amount of temporal punishment, or penance. In another way one might say that the penitential suffering is overwhelmed by good works, which could possibly be a positive from an eastern perspective.
In the early church penances could be quite severe, some individuals were denied the Eucharist for years. Others had acts of public penance imposed upon them which subjected them to public ridicule and scorn on the street. In an environment like that an indulgence could be seen as a form of mercy being applied by giving the repentant sinner other options.
The notion of Purgatory apparently became associated with this practice because (in the Latin tradition) temporal punishment which has not been satisfied in the earthly life must be satisfied in the next. Most people probably assume that they will be far from perfect at the end of their earthly lives and thus an indulgence will ultimately reduce the backlog of punishment in Purgatory.
For several reasons, to me this can seem an alien concept to an eastern Christian. First, the notion of continuing “punishment” for a person whom will be certainly saved. Easterners will not think of ascetic practices as punishment as much as they will think of them as exercises.
A second aspect that may be unfamiliar to an easterner is the transactional nature of Indulgences. Different practices are considered to be equivalent and can be ‘swapped’, ie: do a novena and receive an indulgence. In the eastern church one will engage in ascetic practices for ones betterment, the thought of measuring the value of this practice does not occur. It can seem like a form of accounting applied to spiritual practice: debits and credits balancing each other out.
One thing I think I should point out is that most Roman Catholics I have known do not pay any attention whatever to indulgences, and many don’t really understand them very well. Besides which the giving of penances is a waning practice in Roman Catholicism at present, many priests do not assign penances in confession routinely. This can make an odd situation for an individual who may wish to take advantage of indulgences but rarely gets assigned any penance against which to mentally associate the practice.
I hope that helps.