That is not what that button is. That button shows the original total and if you make any split payments etc it will clear it back to original total.. To enter entire amount use the All button. That is not a fast payment button.. Fast Payment button is something else entirely and its made through automation.
Example of use of the button in question. Type 2 and press split.. it will show remaining amount... press the button in question and it takes it back to original .
EDIT: Actually im wrong... i am not sure what that button is for. All is what selects total amount...That button is behaving like the x. @emre any idea?