Actually, it turns out that I wasn't credited $16.05 at checkout. I was charged the entire $16.05 amount to my credit card, and received no $15 credit in lieu of what was previously the free continental breakfast benefit, as a Diamond. I am now left to argue it out with the hotel, from a distance.
If I could do it again, I would vigorously interrogate the front desk at Hilton Back Bay during check out to ensure confirmation that the $15 credit had been applied. I would ask for proof in writing, with the credit clearly indicated. I would not walk away and assume what they told me verbally was actually true.