There's nothing stopping you from charging money for FOSS software. Sell the first one for as much as it cost to write it.

I assume you mean "sell it before you open source it."

If you found one willing buyer while it was still closed source, odds are there are others. Why rob yourself of additional income? Open sourcing does happen from closed source, but usually that's long after the income stream has exhausted itself (see, e.g., https://github.com/microsoft/ms-dos).

There's nothing stopping you, true. But I think you will find that nobody is going to buy software for whatever thousands of dollars it took you to make it. So the limitation in this case is a practical one, not a legal or moral one.