Thanks for the quick respone!
I did a lot of reading about how duplicati works, given the resources you provided and I thought of a way to solve my problem.
Having read this:
I understand that only one process is responsible to get all the files wich are ready for upload and queue them to be uploaded simultaneously. Correct me if I have any misconceptions.
Wouldn’t be possible to check the quota size before queueing a file to be uploaded and update the quota size for the next file to check? No need for any synchronization as the task is performed by one process.
As of the operations that are needed to be done if the quota is exceeded is a problem for later. 