Hi paddyr , sorry I missed your reply. AccountRIght is entirely date driven, so a transaction passed through with a July date will be reported in July. (If that period is locked in AccountRight at the time of posting, an error will be generated. It's possible that Workbench handles that error and changes the date to the current month (unlikely but worth asking WB).)
The rule shown does not apply to AccountRight, which only has one date.
So when you say that "MYOB only supports the Posting date and not the Invoice date.", this implies that WB is passing the posting date not the invoice date. That is just plain wrong. Please refer to WB support. It may be that a configuration switch is incorrect, or possibly a bug exists.