If you specify a descending sort order based on your Billing Year_Month field in the relationship between these two tables, then you can place the Transaction_Count field from Transactions on your layout and it will show data from the most recent related record.
You can specify a sort order at the relationship level if you go to manage | Database | Relationships and double click the relationship line linking the two Tutorial: What are Table Occurrences?.
Yes! Thank you! That worked like a charm! So simple...