A chump is a stupid person. I don't think you can use the word, properly or traditionally, as a verb or an adjective, but in these informal constructions, it would be understood as you have suggested in the first instance (deceived, taken for fools/chumps).
The second use is less clear. Stupid change? I think the writer means to say, "It's not insignificant change" - he may have bought a candy bar with a $100 bill and forgot to get his change? Oh, come to think of it, that is pretty stupid. Chump!
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