By using ORM method create, you can provide conditional sequence to any field and also by using the condition, we can even select which sequence to load for a particular field, so for your case:

def create(self, cr, uid, vals, context=None):
        if vals.get('chalan')=='Chalan':
             if vals.get('chalan_number','/')=='/':
                 vals['chalan_number'] = self.pool.get('ir.sequence').get(cr, uid, 'your_sequence_name') or '/'     
             return super(your_class_name,self).create(cr, uid, vals, context=context)