I found a workaround.

I just added a property to the order before creating the payment, like so: 

 <xsl:value-of select="CommerceLibrary:SetOrderProperty('pageId', $pageID)"/>

Then on the postback from DIBS, I retrieved the order by its orderGuid. Then checkedout the order and the redirect the page with javascript:

 <xsl:variable name="theOrder" select="CommerceLibrary:GetPurchaseOrderByGuid($guid,false)" /> 

 <xsl:variable name="thePageId" select="$theOrder/purchaseOrder/orderProperties/orderProperty[@key='pageId']/@value" />

 <xsl:value-of select="CommerceLibrary:ExecuteBasketPipeline()" />

 <xsl:value-of select="CommerceLibrary:Checkout()" /> 

  <script>

  function redir(){

window.location = '<xsl:value-of select="umbraco.library:NiceUrl($thePageId)" />';

}

redir();

 </script>

:)