If so, do I report it as income to the IRS? Are there security-cleared accountants who can help me do my taxes? Are there security-cleared IRS tax agents who process my taxes?
This is a really interesting question — and, as it turns out, a really difficult one to answer accurately. None of my usual go-to works on intelligence history dwell much on this kind of operational bureaucracy. I did, however, find this interesting essay on handling double agents, published in the CIA's in-house journal, Studies in Intelligence, in 1962.
The author sets out a list of 'do's and don'ts' for case officers handling double agents, including this advice:
Require the agent to report and, as security permits, turn over to you everything he gets from the other side—money, gifts, equipment, documents, etc. If he is permitted to hold out anything he may grow confused about which side he is working for. But do not be too rigid in following this rule. It may be better, for example, instead of confiscating his payments from the adversary, to put them into a third country bank account and promise him the lump sum upon successful termination.
It doesn't directly answer your question, but I also found a relevant snippet from the memoir of a former FBI and CIA counterintelligence officer, Christopher Lynch. He writes that double agents on the FBI's payroll in the 1970s and 1980s were indeed expected to hand over to the Bureau any payments they received, and received reimbursement through separate channels — the rationale apparently being that this prevented those agents from technically violating federal espionage law:
I was put in charge of keeping track of payments foreign services made to our double agents, going around to the responsible desk agents every three months to update any transactions. Many of the cases I had to keep track of were inactive, and if an operation ended with money in the account, eventually it would go to the U.S. Treasury. I learned quite a bit about how we handled double agents as part of this process, including the legal ramifications of our convoluted financial transactions. The double agents gave us all the money they received from hostile services, and we paid them from separate accounts. Because, by definition, that was espionage.
I haven't encountered Lynch or his book before now, so I don't have a good read on his wider context or credibility. And as to the tax question, I'm afraid I've got nothing meaningful to offer there.
As a follow up question; what about from the east German/Soviet side and what would have been the medium of exchange?