Huh, i don't know which place of Europe you're thinking about but this seems highly unlikely. You can't exceed 48h/week in the EU, and 100$/month (84€) is just stupidly low for countries in western Europe, it would even be illegal in a lot of them: For example, in France, an internship longer than two months has to be paid 3,9€/hour a minima (for reference, minimum wage in france is 10,25€/hour).

I was thinking eastern Europe.

And for professional jobs aka "Cardere" as they are called in France the WTD doesn't really apply

Yeah, and probably these companies also depend on the student having the government's free public transport card

It certainly wouldn't be legal to dock an employee's wages in any central European country. Labor laws here are too good to allow that kind of abuse.