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The fine was still a part of your sentence because you weren't resentenced on your violation of probation. The sentence you received the second time was not a sentence on the original crime, but rather a new sentence on the violation of probation.
That being said, there is a six year statute of limitations on collecting old debt in Pennsylvania. Consumerist.com suggests that you send a certified letter disputing the debt and asking for verification that the debt is legal and not time-barred. It says that at that point, the collection agencies usually give up.
The statute of limitations applies to any entity, including the government. Especially now that the old debt has been bought by a collection agency.
Here's an article about what to do with these collection agencies that are trying to collect old debt that might help you out:
And here's information and a link to a sample letter that you can tweak to fit your situation: