I hope there is an account here tonight....?

I have a question about wages, ok this is just an example, say you earned a monthly wage of �1000 which you should be taxed on, ok then say you have �300 mileage which is none taxable, right now say your employer deducted the �300 from the �1000 so you wouldn`t have to pay so much tax, is this legal for them to do that? surely you should be taxed on the �1000 then what is left added to the �300 then that would be your bring home pay?! could some one please tell me if this is legal, thank you