Xero
Once you "lock" a period in Xero, we can no longer post any journal, invoice or bill as "authorised", we can only post as "draft". This is the way the Xero API has been designed, and is to protect the integrity of your Xero file.
If you need to post something from Mayday into Xero after the period is locked, you can either post it from Mayday as draft, then unlock the period in Xero and complete the posting. Or simply unlock the period in Xero, then run the posting from Mayday.
QBO
We cannot post at all in to a locked period in QBO. If you need to post something from Mayday into QBO after the period is locked, you need to simply unlock the period in QBO, then run the posting from Mayday.
We understand at times you may lock a period and then have to unlock it later to account for a late transaction. When this happens, and that late transaction would have been relevant to a recharge calculation, we will notify you in product. You can read more about this in this article: How to account for late transactions