With jQuery, you have to access the contents of the document of your child window:
$(purchaseWin.document).ready(function () {
$(purchaseWin.document).contents().find('#tdProduct').html('2');
});
Without libraries, with plain JavaScript, you can do it this way:
purchaseWin.onload = function () {
purchaseWin.document.getElementById('tdProduct').innerHTML = '2';
};
I think that the problem was that you were trying to retrieve the DOM element before the child window actually loaded.