Simple math.
Your money in total is 100 you'd borrow from sherwin and filina(50 each)
umbrella cost 97.
So.
100 - 97 = 3
You decided to pay sherwin and filina 1 peso each.
So.
Sherwin 50 - 1 =49
Filina 50 -1 = 49
Add
49 + 49 = 98
There is no missing peso.
Remember the 3 pesos were change from the umbrella, so instead of adding it to the 98, you should have subtracted it.
98 - 1 = 97
That means you still owe Sherwin and Filina 48.5 each.
It's tricky if people will think that they should add the 1 peso to 98. -_-
I hope it help.
