With the girl you mentioned specifically, the right move is. . . no reply.
As you know, if you reply to what she said. . . then the ball is no longer in her court. Give her radio silence for a week. Because you're a busy lothario, who has other girls to seduce. She might change her tune during this time, and hit you up to give you her schedule.
Or if you don't hear anything, she'll be wondering if she lost her chance with you. Then after you can hit her up with a "Hey" and after her reply "Still on for that coffee / drink / whatever you invited her to."