Permit to Engage Activity Other than Permitted but no Salary Raise

Someone balance me here. I'm an ALT on an instructor visa. I recently covered someone's Eikaiwa lesson and my dispatch claimed the kids enjoyed it.

So they offered to get me a permit to engage in activities other than permitted. I agreed, assuming it would come with some extra income.

Then they sent the contract, after asking for my passport and residence card and filing the application.

There's no salary change.

What should I do? Will this have tax implications on me? Can I break contract and cite this as a reason?

TIA

Edit: Eikaiwa lessons run from 4.30-9.30 pm and also weekends.