Both the Mamiya RB 150mm SF and the RZ 180mm SF lenses sell for dirt cheap right now (I recently sold a boxed mint- example of the 180 for about $100.00 on Ebay). There doesn't seem to be much interest in them for some reason. I don't understand it because soft focus is all rage in large format and SF lenses go for a premium.
You would probably be better off returning it and buying another. Just my 2 cents.