Letting Agent Asking Contractor to add 10% Fee to his Invoice and then they take it off again.

Basically i am having problems with letting agents asking me (I'm a contractor for them) to add a 10% fee onto my invoice to them. I then have to show a deduction and take a fee off to bring the money back down to the original figure. How am I meant to invoice them please? Surely this can't be right? Also then the figures don't match up as adding 10% on to a figure isn't the same as when you take it back off? Also with adding on 10% to my price each time I do an invoice will obviously make an impact on my turnover values falsely because I'm not actually adding anything, what can i do about that please?

Hi,
This is not a subject that HMRC can advise on. This is between you and the agent.
Thank you