Your understanding of this is not entirely accurate.
You can withdraw your available money at any time. It does not matter if it is $100 or more.
What may have confused you is that there is an option to have money AUTOMATICALLY withdrawn to your withdrawal account when the available funds reach $100 or more.
If the amount is less, you can click a button and MANUALLY withdraw the money, no matter how much it is (or as long as it exceeds the amount of the fee).
For example, two days ago, at 5:00 p.m. (my time), an $80 payment (for a fixed-price contract) became available. I went to my Reports page, clicked a button, and withdrew that amount to my bank. There are no fees associated with making such a withdrawal, so there's no reason to postpone withdrawing the money.