Your solution won't work and requires a new architecture of OU and Group management.
OU's in a modern Windows AD are only need for delegated admin permissions and IT admin convenience. Don't let the users control your OU structure (how many moves/renames do you do based on div/dept changes?) Sure OU's can be used for lots of other things such as a Organizational Chart but your problem highlights a weakness of OU's: They are a linear structure like file system folders. You can't be in two at the same time.
A short term fix is to create a new manual Distro Group and add the OU-based dynamic group and Bob to it. Make the new manual the name of the old OU-based dynamic group so users "may not" notice the difference. Their Outlook auto suggest may cause problems though.
Another option is to create a new user account in Finance with an autoforward rule to Bob's mailbox. Call it something like "Autofoward 001" or something generic so people don't email it thinking it's a real person. This option is cleaner then the first.
Long term you need to base your Dynamic Distro Groups off AD attributes and not OU's. Or better yet IMO, make those org groups manual, then assign 1-3 people in each department as owners of the distro group who have responsibility to their dept leaders to keep the group current. This delegation of authority keeps it off your IT admin plate and allows anyone to come in and out of those groups regardless of your OU design.
People will debate that OU's are for more then IT Admin management and delegation but often what people use OU's for can be done in a more flexible way with all the modern GPO features.