I am using Zoho Books Professional plan. If I subscribe to Zoho Finance Plus, what will happen to my Zoho Books subscription amount?

Based on your usage/duration of Zoho Books, the prorated amount will be calculated and adjusted with the Zoho Finance Plus subscription.

If there is any excess amount post adjustment, then you can request a refund by dropping a mail to support[at]zohofinanceplus[dot]com.