Hmm. Not a very helpful letter when all said and done. Why did they put in that statement about final pay.

If the employer can't change, update or make a new letter you'll need to be sure to give an explanation.

Anyway, as long as the payslip & bank statement evidence can be submitted it should be OK, just a little risk.
Try to have the letter updated, just to reduce risks.

Also take a look here at this thread to see how the income is calculated and reviewed.
Salary rules for fiance visa?