How to create an PayPal button with overwritable variables

Question

Hello I would like to create a paypal buy button which has a dynamic set amount. I would like to pass the amount by a text input field within the form and the item_number by a hidden field.

The issue is that what ever I do I get a encrypted s-xclick button from the paypal website. This button does not allow hidden variables being placed in the form.

I think what I need is a xclick button. My goal is to allow users to increase their internally credit of my website.

EDIT (moving the addition to the question from the answer to the question)(from here @tokam:

To add this to the discussion I would like to show my current solution for the problem:

Here we have some Javascript validation which helps the user with the input. Recognize that it opens a lightbox on success

function validatePaypalForm()
{
    var val = $('#paypalPaymentAmount').val().replace(/\s*$/, "").replace(/,/ , ".").replace(/€$/, "");
    var errormsg = '';
    var ret, amountField;
    if( val==='' || isNaN( parseFloat(val) ) || !isFinite(val) )
    {
        errormsg = 'Bitte geben Sie einen gültigen Betrag an';
}else if( parseFloat( val ) < <?php echo $this->minimum?>  )
{
    errormsg = 'Das Einzahlungsminimum betr&auml;gt <?php echo $this->minimum?>&euro;';
}

ret = ( errormsg === '' );


amountField = $( '#paypalAmountField' );
if( ret )
{
    amountField.removeClass( 'error' );     
    $('#paypalAmountErrorMessage').html( '&nbsp;' );
    $('#paypalPaymentAmount').val( val );
    fb.start( 
        '<p><strong>Sie werden in kürze zur Seite von Paypal weitergeleitet.</strong></p>',
        'width:700 showPrint:false modal:true showClose:false showOuterClose:true showItemNumber:false closeOnNewWindow:false outsideClickCloses:true innerBorder:0 imageClickCloses:false scrolling: no'
    );

}else{
    amountField.addClass( 'error' );
    $('#paypalAmountErrorMessage').html( errormsg );
}

return ret;

} Here comes my button now. The issues I am having with are e.g. that it is easy for the user to set an other currency code. I could handle this in my IPN Listener by refunding the payment. Are there other issues which come with an unencrypted changeable button?

<form onsubmit="return validatePaypalForm();" class="stn-form" action="https://www.paypal.com/cgi-bin/webscr" method="post">
<fieldset id="fieldset-p"><legend><span>2.</span>myproject Guthaben aufladen per Paypal Zahlung</legend>
<div id='paypalAmountField' class="field">
<label for='paypalPaymentAmount' >Betrag &euro;:</label>
<input id='paypalPaymentAmount' type="text" name='amount' value='' />
<span style='display:block;' id='paypalAmountErrorMessage' class='errorText'>'&nbsp;</span>

</div>
<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="business" value="THE_ID_OF_MY_CLIENT">
<input type="hidden" name="lc" value="DE">
<input type="hidden" name="item_name" value="myproject Advertiser Vorkasse">
<input type="hidden" name='item_number' value="11500">
<input type="hidden" name="currency_code" value="EUR">
<input type="hidden" name="button_subtype" value="services">
<input type="hidden" name="no_note" value="1">
<input type="hidden" name="no_shipping" value="1">
<input type="hidden" name="bn" value="PP-BuyNowBF:btn_paynowCC_LG.gif:NonHosted">
<input type="hidden" name="rm" value="1">
<input type="hidden" name='cbt' value="Zu myproject.de zur&uuml;ckkehren">
<input type="hidden" name="currency_code" value="EUR">
<input type="hidden" name="return" value="http://myproject.somedomain.net/advertiser/guthaben-aufladen/ret/success" />
<input type="hidden" name="cancel_ return" value="http://myproject.somedomain.net/advertiser/guthaben-aufladen/ret/canceled" />
<div class="actionrow">

<input type="image" src="https://www.paypalobjects.com/de_DE/DE/i/btn/btn_paynowCC_LG.gif" border="0" name="submit" alt="Jetzt einfach, schnell und sicher online bezahlen – mit PayPal.">
<img alt="" border="0" src="https://www.paypalobjects.com/de_DE/i/scr/pixel.gif" width="1" height="1">
</div>
</fieldset>
</form>

Answer 1

The reason you can't override the amount dynamically, is because you have a so-called 'PayPal hosted button'.
With a hosted button, the amount is stored on PayPal's side and can't be overwritten with the 'amount' variable. You'll either want to use a non-hosted button, or use the BMUpdateButton API call to dynamically update the button's amount.
To use a non-hosted button, simply find 'Step 2' in the button creation tool and untick 'Host button with PayPal'.

Option 2: Still use the hosted button, and use the BMUpdateButton API to update the amount. An example request for BMUpdateButton would look as follows:

USER=Your API username
PWD=Your API password
SIGNATURE=Your API signature
VERSION=82.0
HOSTEDUBTTONID=The value of <input type="hidden" name="hosted_button_id" value="">
BUTTONTYPE=The type of button. E.g. BUYNOW
BUTTONCODE=The type of code you want to get back. E.g. HOSTED
L_BUTTONVAR0=amount=The new amount with a period as separator
L_BUTTONVAR1=item_name=Optional: a new item name if you wish

Similary, you could also use the BMCreateButton API to create a new button, or use the BMButtonSearch API to search through a list of all your stored hosted buttons (to find the hosted_button_id of your button automatically, for example)

The reason to use a hosted button is because it's more secure. A non-hosted, unencrypted button would basically leave the amounts open to manipulation. Fraudulent transactions waiting to happen.