Can I deduct the £66.15 SSP from the wages of an employee because we have a company sick pay scheme that pays more than SSP?

Basically can we claim the £66.15 SSP and then take it off them, but then give them company sick pay (CSP) which is based on the average hours the employee worked last year.

e.g. £66.15 (SSP) – £66.15 (SSP) + £200 (CSP)

I am asking this because my employee is claiming that this is an illegal deduction from his wages because he has not agreed to this.
Bruce