I found the reference to "- 1%" to be confusing. At first I was trying to take 1% of the remaining balance.

This sentence made it clear: "+ 1% from the withdrawn cash plus the robots cannot go beyond the card’s balance."

Perhaps the comments in the code can be updated to reflect this such as:

120 - 10 - 0.5 - 1% (10 * 0.01) = floor(109.4) = 109