How many ways is the assumption wrong?
Shmitta deals with chovos. Nothing to do with status of Eved Ivri. He goes free at Yovel, not Shmitta. The only chovos that are eliminated are those with collection pending. An eved who is sold to pay the debt, the debt is settled with the purchase money. There is no further collection. He then has to serve out the term of his avdus, which is 6 years, regardless of when in a shmitta cycle it falls. A loan with mashkon is exempted for this same reason. It is not collection as much as it is trading payment to get your collateral back.