For exchanges, Rich Returns allows store owners to either create a (real) order in Shopify or a draft order. The draft order option does not reserve inventory for the new item until it is converted to a real order in Shopify.
The option in Shopify's dashboard to Reserve items will NOT reserve inventory according to merchant reports in the Shopify forums, see e.g. https://community.shopify.com/c/Payments-Shipping-Fulfillment/What-is-the-point-of-Reserve-Items-on-a-Draft-order/td-p/454374
Shopify does not offer functionality of reserving items for draft orders to developers, yet. So the only option Shopify allows at the moment is to either create a real order (deducts inventory) or a draft order (does not deduct inventory).