Fix precision of price returned from price entry dialog

The returned price was truncated by the global price precision which
could cause all kinds of trouble when an amount was entered but the
price precision was not large enough to yield the same result in the
equation result = amount / price * price;

Now the price is returned without precision loss in case it is based
on an amount (method is selected by the user in the dialog). In the
case the price is entered it is returned with the global price
precision in effect.