Let's say that a country is adapted to parecon. An individual, one very skilled tailor, finds that people appreciate his work, and that he can charge them for favors. Soon, the tailor has made such a profit that he can give pay to other tailors to do his work on a larger scale, and after a while he has established an industry. With his profits, his empire can freely expand.
Capitalism is reinstated.
One can imagine lots of different approaches to the practice in different parecons. The second part of the problem, hiring wage slaves with lucre gained from black marketeering, is simply not an option in parecon. Make the problem more real. A country goes parecon, another hasn't. A rich person from the latter comes into the parecon and advertises for wage slaves to work in a factory of his, that he wants to build in the parecon country, at a high pay rate, let's say.