Any payment for doing a stage in France?

This depends on how long the placement is.

A payment, technically described as a gratuity rather than a salary, becomes obligatory if the stage is longer than two months in the university year.

Note that a stage usually refers to a placement in a workplace that is complementary to academic studies.

This could refer to more than 44 days at seven hours a day, or more than 308 hours of work at different amounts per day.

In other circumstances, the employer is free to pay or not.

The standard pay is 15% of the plafond de la sécurité sociale, a sum used in calculating various entitlements, which works out at €3.75/hour – a modest amount considering the Smic minimum wage is €10.03 before social charges (€7.72 afterwards) but it is free of charges.

With a few exceptions depending on work sector, if the employer pays more, anything above this is subject to charges. It is not subject to income tax.