The new "Pay some amount" mechanic

This event has a new mechanic that I've not seen before in an event.  I have taken some screenshots to help make it easier to understand.  This first pic shows what the window looks like when you click on the quest to view it.

The red arrow points to a plus sign, that I am assuming will either be green (if you can afford it right now) or possibly gray (if you can't afford it).  If you click on the green plus sign, you will get the next window.

Once you get to this screen, you have two choices on how to pay.  You can click the "Maximum" button on the right side which will automatically add whatever is required.  Or you could click the green plus sign to increase the amount to whatever you want (say, if you didn't want to pay it all right now for some reason).  Either way, once you are done, then click on the "Pay" button which should be green once you decide how much you are going to spend.

Just a new mechanic, and easy to use once you figure it out!