According to the Biographical Film Too Big to Fail , it was the British and not Paulson who killed the Barclays Deal. They were pretty close to a deal and actually had all the other major banks buy up Lehman's toxic assets and Barclays was about to agree to the deal but the British Regulators killed it.
I think the British regulators required the US the guarantee Lehman's toxic assets, which they could have done and would have likely made a profit on them later on.
Barclays wouldnt have bought it with the toxic assets though, thats why Paulson called all the top banking CEO's to the Fed to come to deal where they would have bought the toxic assets. If you are talking about that guarantee according to
To Big to Fail the CEO's had come to an tentative agreement to buy the toxic assets.