Ensure transfer update is done under one transaction

Let's say that we create a manual transfer and the scheduler starts and call the existing code to check the status (the exiting service will check the status and if the files are uploaded he will update the files on D.B Not the Nuxeo documents. After that it will save the transfer.
Then, the scheduler is still working and a user refreshes his UI and sees the button Complete available and he clicks on it. We probably get a race condition Not allowed to follow transition <to_started> from state <pending>  I mean the scheduler try put back the transfer to Start  state but it's already on Pending.