Is this deduction, right?

I work for a client company through a haken agency and I am paid hourly. Recently, I negotiated a raise directly with the client company, and they agreed to increase my hourly rate by lest say ¥250. They then discussed it with my haken company.

However, my haken agency has now sent me a new contract showing only a ¥200 increase, saying that the remaining ¥50 will be used to cover additional taxes and pension contributions that they need to add because of the raise.

I’d like to know whether this is actually correct.