There are invoice ID's and Invoice Numbers and they should not be treated the same. The starter system uses an auto-entered serial number to be used as an Invoice ID to link to line item records. I would not change this to turn the ID code into a "number".
Numbering systems such as you descibe are often used in invoicing systems for auditing and other purposes. This should be set up as a differnt field and not used to link to other tables.
Define a new serial number field, InvSequence, for XX. You can manually reset it's next serial value to 1 at the start of each new year or you can write a script that checks dates and settings every time the file is opened and if it determines this is the first business day of a new year, resets the serial number for you.
Define a text field with the following calculation for your Invoice Number:
Let ( Today = Get ( CurrentDate ) ; InvSequence & "/" & Right ( "0" & Month ( Today ) ; 2 ) & "/" & Year ( Today ) )
Create field Serial#
Set Serial# to auto-enter serial
Create a field InvoiceID
Set it to auto-enter by calculated value
Enter the formula: GetAsText(Serial#) & Filter ( Middle (Get (CurrentDate); 4;7); 1234567890")
Edit: Ignore my answer. Phil's solution more robust. Beat me to it
Thanks, it works Phil.