Inventory sync deals only with the OpenCart and Square product quantities. It ensures your OpenCart and Square inventories will remain in sync.
Sync Direction (Square ⇒ OpenCart)
The Square extension can only do Inventory Sync in the direction Square ⇒ OpenCart. This means that you must not modify product quantities in OpenCart. Instead, you should use the Square dashboard to modify your inventory. Any changes will be pushed to OpenCart automatically in a matter of seconds (or at most, a minute).
In case an OpenCart product is already synced with Square, you will see a warning in the product edit page in OpenCart:
Inventory Sync is performed automatically every 15 minutes.
Enabling Inventory Sync
Inventory Sync only works on OpenCart products which have the setting Subtract Stock set to Yes. In case a product has required Select or Radio options (for more info see Catalog Sync), their option values must also have Subtract Stock set to Yes. Otherwise, Inventory Sync will be disabled for this product.
Setting all Subtract Stock options to Yes in OpenCart will enable Inventory Tracking in Square:
Likewise, enabling Inventory Tracking in Square will automatically update your OpenCart product (and required Select or Radio options) by setting Subtract Stock to Yes.
Inventory changes from OpenCart
The Square extension ensures all inventory made by other payment extensions will be pushed to Square. For example, if an OpenCart item has been purchased via PayPal, the Square extension will detect this inventory change and account for it in the Square inventories.
The same occurs for any re-stocks performed within OpenCart as a result of a refund.
Disclaimer In some cases, the Square extension may not be able to detect inventory changes done within OpenCart, particularly if these changes are made by third-party modifications/extensions. In such cases, additional work will need to be done to alleviate this issue.