Look, Kase has been anything but subtle and tactful in this arc and everything else in this chapter was her bungling around so I don't think she managed to change the terms of the bet. I read it as her adding a new bet to the race. Now being very generous you could view this as her way of maybe convincing her roommate to let her move out even if she loses since she'll then explain she is moving in with her "boyfriend". So essentially this second bet could be a last ditch shot in the event she loses. Still I think that is giving way too much credit to Kase.
I agree with the others on this coz she said if Fukami can beat her in a 100m race then she'll tell her the reason she's leaving - meaning she's leaving no matter what. She's only going to give Fukami the reason she's leaving if she loses but won't be staying. No point in having 2 races and the bets would be contradicting if both bets are in play.
