Passing Build Back Better in the form that the progressives want will pour a lot of gasoline on the fire of inflation. That is basic Economics 101.
Economics 101 says "I don't really know anything about economics but here's a throw away comment to make me appear informed", or "I'm a Republican who knows nothing but think I know it all because I listen to rich people". Inflation isn't something that goes along with spending automatically but a reaction to supply side problems, such as a global pandemic might cause. The price increases seen in most goods are a "following" event to such problems and not a result of too much money.
Why? Because he is the Grinch? No, because he is concerned about inflation. I wish a few more Democrats were equally concerned about it.
Manchin isn't actually concerned about inflation. He's concerned that the bill might tip the power scale toward workers just a little instead of continuing the slide toward capital that the last 50 yrs of trickle down have given us.
He, probably better than most in government, knows how money is created and how it impacts society. He knows that Congress can "afford" anything that is available in the private sector and priced is US dollars, even without revenue. He understands the language of the oligarchs because he r one. Fear of inflation has been a political ruse, much like "pay fors", since we ditched the gold standard for a modern economic system.
In his defense (sort of?) he is no worse than any moderate Democrat, including Sanders and Biden, for pushing taxation as a prerequisite to spending. The threat of such taxation is how Democrats blackmail their donors and the reason they lose so damn often after they repeatedly fail to deliver anything to voters.