Perform a sparse update on a QuickBooks Online Invoice with SyncToken concurrency control

Steps

1. Retrieve the current Invoice using the QBO REST API to obtain the latest SyncToken value; this token changes on every write and is required to prevent conflicting updates.
2. Construct a JSON request body containing at minimum: the Invoice Id, the current SyncToken, the field(s) you want to update, and the property "sparse": true at the top level of the body object.
3. Send a POST request to the QBO v3 Invoice endpoint (e.g., /v3/company/{realmId}/invoice) with the Content-Type set to application/json and the sparse body.
4. Check the response for a successful updated Invoice object; the response will include a new SyncToken which must be used for any subsequent updates.
5. If the server returns a stale SyncToken error (business validation fault), re-fetch the Invoice to get the latest SyncToken and retry the update.
6. For production usage, implement optimistic concurrency handling so concurrent writers re-read and re-attempt rather than silently overwriting each other's changes.