I thing they are feeding you a BIG line.. The belt is about $55. The belt tensioner should not be affected by getting any oil on it. It didnt hurt mine when I blew out out the dipstick tube. It is just a spring arm mechanism with a pulley on it. Not sure what 'seal' they are talking about unless it is the oil pan gasket.

Tell them to fly a kite and find yourself a good auto shop.

MikeR