from the products insert page in the admin folder, delete the following 2 lines:
var sprytextfield4 = new Spry.Widget.ValidationTextField("sprytextfield4", "real", {validateOn:["blur"]});
$WAFV_Errors .= WAValidateNM(((isset($_POST["ProductPrice"]))?$_POST["ProductPrice"]:"") . "",0,1000000,2,",.",true,4);
Please Note:
This is not a modification we can offer support for, power store does not support free items, this may cause issues when going to the gateway if only the free item is in the cart, the gateway will not process a transaction for $0