Fundraising - Manual Donation Not Reflected in Reward Limit

I want to engineer some social proof into my campaign by adding manual donations that increase the number of Backers and $ amount raised, while simultaneously reducing the Reward Limit Count on the individual reward ...

$100 - Limited (8 left of 10)

How can I changed the number remaining (e.g. 8) without changing the total number that 'was' available? If not possible, this seems like it would be a very useful option.