Im not sure what the point of the scenario was, but person c...

Im not sure what the point of the scenario was, but person c bought shares on market.

person A has 850 on register, and the shorter owes them another 100
person C has their 100 as bought normally

the shorter can close out with person A by either buying back on market and then giving them the shares, or if agreed, by giving them cash for the market value of shares.