If they are open to negotiation, you may suggest that they "gross-up" the payment of the lease. This is a common strategy used when the employer desires that the employee receive the full cost of a benefit that is reduced because of income and payroll taxes.
When an amount is grossed up, there is an equation used that takes into consideration the employee's federal and state tax brackets, and adds the SS and Medicare %'s. Then, when this "grossed-up" amount is added to the employees income, he or she will realize the approximate value of the benefit.
For example, if you are in the 15% bracket, and pay approx. 10% SS, Medicare and state tax, and you receive a taxable benefit of 1000 dollars, you only see 750. So the employer would need to pay you approx 1332 dollars for you to realize the net benefit.
You could argue that your "agreement" does not state that you will be paying tax on the lease payment, and that you are prepared to dispute it with the IRS, and possibly with an attorney. Then you could suggest grossing up the lease payment as an "out" for them.