Hi Checco and welcome to the forum!
I'm not exactly sure what you mean by "free", but I think you are talking about setting the xsi:nill attribute, which can be done by changing your XML string to the following:
<my:Prestito xmlns:my='http://schemas.microsoft.com/office/infopath/2003/myXSD/2007-01-18T17:05:24' xmlns:xsi='http://www.w3.org/2000/10/XMLSchema-instance'>
<my:Utente></my:Utente>
<my:DataPrestito xsi:nil='true'></my:DataPrestito>
<my:DataRestituzione xsi:nil='true'></my:DataRestituzione></my:Prestito>
I think this will solve the problem - good luck!
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge