My Employer Has Been Under Paying Me 3.5% Of Pay. Is This Legal?

This seems completely illegal to me and I would appreciate opinions.

I essentially work on commission and collect 50% of all billing’s I render, and this is stated in my work contract.

My employee has been taking off 3.5% of pay every pay cheque under the notion of it being “receptionist coverage”. However, no where in our does it say that the employer is allowed to do this. We agreed I would get 50%, but instead I have been getting 46.5%.

Every time I talk to my employer they say “we are looking into this and are trying to remove it”. But they have been saying this for 6 months.

I feel like this is not right and I should get the back pay for 6 months.

Thoughts?