I had exactly the same issue at the Marriott Northwest in Hoffman Estates IL. When I gave the desk clerk my Premium Pounds Cert she said she knew what they were and not to worry about it.
Later I checked my account via the TV in the room and saw that my account was credited with only $100 not the approximately $170 that I expected.

I went to the front desk immediately and spoke to the hotel manager. He said he thinks they were worth more but the "accountant" said they were worth only $100. When I asked him to get the cert he couldn't find it at first until he saw I was not going to go away, I either wanted to switch it with a Bonus Buck Coupon or at least make a copy of it (I always make a copy now). He finally found it and it had already been voided so I just had him make a copy for me. I was very surprised that the hotel manager couldn't over ride the "accountant".


The issue was resolved with a letter to Marriott Customer Service and my credit card was refunded the difference about two months later.