Changing Control Values From a Child Window??

Is it possible to change a control value from a child window?

I have a child window that needs to complete a couple of functions, post the data back the parent window and self close. I've achieved this except changing the control value of a AW.UI.CheckBox and cant work out how??

Here is my attempt that also includes posting some data back to a three HTML inputs that works but the CashSales.setControlValue dosent??

function updateParent() {
var a = PayTotal.getControlValue();
var b = Balance.getControlValue();
var c = Change.getControlValue();

opener.document.SL_Invoice.Pay.value = a;
opener.document.SL_Invoice.Bal.value = b;
opener.document.SL_Invoice.Cha.value = c;
opener.document.SL_Invoice.CashSales.setControlValue(false);

self.close();
return false;
}

<input type="button" name="Ok" Value="Post" class="Ok" onclick="updateParent();"/>