Will the Offers, Discounts and Pricing Management plugin deactivation affect my existing subscription?

Will the Offers, Discounts and Pricing Management plugin deactivation affect my existing subscription?

Yes, deactivating the Offers, Discounts and Pricing Management plugin will reduce your subscription amount for your next billing cycle. The reduction in amount will depend upon your choice of edition, number of plugin licences, and the billing cycle that you have opted for. The Cart Details on the Purchase Subscription screen clearly indicates the subscription amount from the next billing cycle. 
Plugins in Tagrain are either Subscription Based or License Based. So the price that you pay for your plugins is based on the number of POS terminals that you have in your setup and the number of licences that you have acquired.
There will be no change in your subscription amount in the current billing cycle if you are only deactivating the Offers, Discounts and Pricing Management plugin AND not updating your Store / POS count. 
Once activated, you cannot deactivate the Multiple Units of Measure plugin because deactivating the plugin would affect the entire master data and all the transactions across the application leading to lead to rounding off discrepancies.