Change of mind on substitution?

[Scrapper1]

NCAA rules, please.

This happened today. I was R2 during a varsity match. We had low-hanging basketball backboards at each end of the volleyball court. Team A's first hit contacts the backboard. Team B coach assumes that she has won the rally and sends a sub into the substitution area and says, "Sub!"

R1 signals "replay" because he feels that Team A could have played the ball that hit the backboard.

Now Team B coach doesn't want to send the sub in. Should we have:

a) required her complete the substitution since she already made the request?
b) allow her to withdraw the sub but charge an improper request?
c) allow her to withdraw the sub without an improper request?
d) done something else?

Thanks. BTW, the R1 chose (c).