Unfair claim

I’ve been with Discovery for a few years now and I hardly touched my MSA for the most part. MSA which I have paid for and they take the balance every year that I don’t spend. Now they want to claim a portion back this year because I change my scheme with them. I don’t feel that this is right. They can use all the MSA which they have taken and net it off the amount they are claiming is due to them.