I doubt the fuel from the manifold would bet sucked back into the bowser. Once it leaves the bowser it should be assumed to be contaminated and a one way valve fitted to ensure there's no risk of back flow which could contaminate the enitre contents.
I agree that the what you loose in the manifold, you get back by receiving what the last guy lost. That is unless the company is on the fiddle and kijangnim is right. Sounds like a potentially dangerous practice if it is.