Okay I got the answer. In .py file

class MyFields(osv.osv):
    _name='my.fields'
    _columns={
         'home_number':fields.char('Home Number',size=100),
         'same_as_above':fields.boolean(),
         'present_number':fields.char('Present Number',size=100)
    }
def onchange_same_as_above(self,cr, uid, ids,same_as_above,home_number,context=None):
    if same_as_above:
        return {'value':{'present_number':home_number,}}
    return {'value':{}}

In XML file

<field name="home_number"/>
<field name="same_as_above" on_change="onchange_same_as_above(same_as_above,present_number)"/>
<field name="present_number"/>

You want to use the boolean field to behave like a button?

To answer your question: No you cannot do this in XML (only). You need to write some code, and link that code to the field (probably on_change_<name field="">). The linking is done in XML, but the code in your custom python module.

Thanks for reply, I want when boolean field is True, then copy the value . In on_change I can use the values from the database , but I cant use the values that are on views , before saving the form ....