Unless you came up with some other goofy arrangement, you pay back $200 - stakeback plus half the profits.
Imagine if you played and ended up with $120 - Would you really only ship $60, for a $40 loss for the backer and a $60 win for the backee, who only REALLY won $20 with someone else's money?
Obviously that doesn't make any sense. A profit was made, so the backer deserves to see some of it and not take a loss. So you'd send back the $100 and then split the remaining $20.
Otherwise, I wouldn't expect to be backed by that same backer ever again - that's for sure.