I returned home and contacted MasterCard. The customer service representative agreed that something definitely did not ad up and was going to investigate further. She agreed to credit the $100 subject to further investigation but indicated that the whole scenario was very peculiar and why would I visit 2 separate pumps for the same transaction. She also informed me that transactions at the pump put a $100 hold on the card while the transactions is taking place and once completed, the $100 is credited from the actual amount purchased. She indicated that sometimes the $100 is not credited due to a malfunction.
I decided to do some further investigation and found that this is not an uncommon occurrence. See below.
I am sensing a may have a fight on my hands trying to prove I did not make the $100.00 purchase but I will fight this to the end. Fortunately, I am one of the few that saves all my credit card receipts. I am discovering that many do not, not that it would help in this particular circumstance.