One Possible Version:
I’m sorry to say that I cannot go to the bookstore with you on Friday afternoon. I have just found that I have to attend an important class meeting that afternoon. I hope the change will not cause you too much trouble.
Shall we go on Saturday morning? We can set out early so that we’ll have more time to read and select books. If it’s convenient for you, let’s meet at 8:30 outside the school gate. If not, let me know what time suits you best. I should be available any time after school next week.