I'm writing a script for a list display. The user puts the cursor on a certain list line and presses a button. Now my InputScript reads the list line and starts a certain transaction. The problem is that the list line is sometimes the wrong one. Only if the user first hits the "Enter" key after putting the cursor on the required line, and then presses the button, does it take the right list line. I'm using the notation 

Pushbutton ...
  using PAR = "&V[_liststring]"

What can I do?

With the notation that you are using,  "&V[_liststring]" is replaced with its value as soon as the screen is displayed. But you need the value after the user has put the cursor on the list line and hit the button. 

To achieve this, do not pass a parameter to the InputScript. Instead, use a command 

Set V[current_line] "&V[_listline]" 

at the beginning of the InputScript, before the first "Screen" command. The same applies for the variables  V[_listline] and V[ _listcursorrow].