There is no missing euro.
You borrowed two times 49 euro = 98, and spend 97 , and now have an extra pound to keep for yourself.
You are adding up what you owe, plus what you have which doesnt work, as there is no 100 . Once you give them the 1 back if you are adding up what you spent you should either add up 49 +49 = shirt + 1 change or if you are adding up what you borrowed originally that is 50+50 = shirt +3 change.
They are two difference equations cleverly put together to make it look like there is a missing pound.
Thanks google.