In general, if you are thinking of the hundred dollars as a collection of individual bills, then the sense is that they are plural, and you use "were".
If you are thinking of the hundred dollars as the value, the quantity, then it is singular, and you would use "was". In this case "of the money" suggests to me that you mean the value, and I would use was.
There is a closely related longer thread on a similar question: one thousand dollars are/is. I think you will find the discussion interesting.
I am leaving this thread open, in case people want to say something about the use of singular or plural in this particular sentence.
'was': We usually use singular with any kind of quantity made of numeral + plural unit.
A hundred grams is enough in this recipe.
Ten kilometres is a long way to walk.
Five days was a long time to go without water.
With plural, 'five days were' would be talking about each of the days separately: five days were boring, three others were interesting. These five days then might not be one block of five days together. A block of time, weight, or money is conceived as a singular thing.