diff --git a/addons/purchase/purchase.py b/addons/purchase/purchase.py
index a3135935045..70ecd495e8c 100644
--- a/addons/purchase/purchase.py
+++ b/addons/purchase/purchase.py
@@ -705,9 +705,13 @@ class purchase_order_line(osv.osv):
         """
         if not uom_id:
             return {'value': {'price_unit': price_unit or 0.0, 'name': name or '', 'notes': notes or'', 'product_uom' : uom_id or False}}
-        return self.onchange_product_id(cr, uid, ids, pricelist_id, product_id, qty, uom_id,
-            partner_id, date_order=date_order, fiscal_position_id=fiscal_position_id, date_planned=date_planned,
-            name=name, price_unit=price_unit, notes=notes, context=context)
+        context.update(uom_change=True)
+        res = self.onchange_product_id(cr, uid, ids, pricelist_id, product_id, qty, uom_id,
+              partner_id, date_order=date_order, fiscal_position_id=fiscal_position_id, date_planned=date_planned,
+              name=name, price_unit=price_unit, notes=notes, context=context)
+        if 'product_uom' in res['value']:
+            del res['value']['product_uom']
+        return res
 
     def _get_date_planned(self, cr, uid, supplier_info, date_order_str, context=None):
         """Return the datetime value to use as Schedule Date (``date_planned``) for
@@ -764,11 +768,13 @@ class purchase_order_line(osv.osv):
         product_uom_po_id = product.uom_po_id.id
         if not uom_id:
             uom_id = product_uom_po_id
-        
+            
+        if not context.get('uom_change'):
+            uom_id = product.uom_id.id
         if product.uom_id.category_id.id != product_uom.browse(cr, uid, uom_id, context=context).category_id.id:
             res['warning'] = {'title': _('Warning'), 'message': _('Selected UOM does not belong to the same category as the product UOM')}
             uom_id = product_uom_po_id
-
+        
         res['value'].update({'product_uom': uom_id})
 
         # - determine product_qty and date_planned based on seller info
diff --git a/addons/sale/sale.py b/addons/sale/sale.py
index 82bfda7b580..81e5fec0856 100644
--- a/addons/sale/sale.py
+++ b/addons/sale/sale.py
@@ -1197,7 +1197,7 @@ class sale_order_line(osv.osv):
         product_uom_obj = self.pool.get('product.uom')
         partner_obj = self.pool.get('res.partner')
         product_obj = self.pool.get('product.product')
-        context = {'lang': lang, 'partner_id': partner_id}
+        context.update(lang=lang, partner_id=partner_id)
         if partner_id:
             lang = partner_obj.browse(cr, uid, partner_id).lang
         context_partner = {'lang': lang, 'partner_id': partner_id}
@@ -1213,12 +1213,10 @@ class sale_order_line(osv.osv):
         result = res.get('value', {})
         warning_msgs = res.get('warning') and res['warning']['message'] or ''
         product_obj = product_obj.browse(cr, uid, product, context=context)
-
+        
         uom2 = False
-        if uom:
+        if uom and context.get('uom_change'):
             uom2 = product_uom_obj.browse(cr, uid, uom)
-            if product_obj.uom_id.category_id.id != uom2.category_id.id:
-                uom = False
         if uos:
             if product_obj.uos_id:
                 uos2 = product_uom_obj.browse(cr, uid, uos)
@@ -1252,6 +1250,10 @@ class sale_order_line(osv.osv):
                         [('category_id', '=', product_obj.uom_id.category_id.id)],
                         'product_uos':
                         [('category_id', '=', uos_category_id)]}
+            
+        if not context.get('uom_change'):
+            res['value'].update(product_uom=product_obj.uom_id.id)
+            uom = product_obj.uom_id.id
 
         elif uos and not uom: # only happens if uom is False
             result['product_uom'] = product_obj.uom_id and product_obj.uom_id.id
@@ -1309,6 +1311,7 @@ class sale_order_line(osv.osv):
             lang=False, update_tax=True, date_order=False, context=None):
         context = context or {}
         lang = lang or ('lang' in context and context['lang'])
+        context.update(uom_change=True)
         res = self.product_id_change(cursor, user, ids, pricelist, product,
                 qty=qty, uom=uom, qty_uos=qty_uos, uos=uos, name=name,
                 partner_id=partner_id, lang=lang, update_tax=update_tax,