Grievances related to fair compensation on assignment to Italy

I am sent to Italy on one year company assignment with an assignment per diem of Euro 2400 per month and company paid rental car + apartment over and above my existing salary in India. The contract also states that any additional taxes apart from what I would have paid on my salary in India will be borne by the company. Now after coming to Italy, I am instructed to use my International debit card linked to my salary account in India for spending the per diem while company insists on paying the equivalent of Euros entitled in INR at an exchange rate much lower than what is charged by the bank for international transactions in EUROs. They insist that per diem cannot be paid in EUROs as they need to account for taxes and the rate used is as per company policy. This causes me an exchange loss of 4-8% on every Euro spent and is exceeding INR 2Lakhs in a span of one year. No information regarding this policy was provided before the assignment in spite of multiple requests from my side.