Who said anything about paying in cash?
I don't think, "How 'bout I buy your car for the $2,500?" constitutes any theft of any kind.
Presumably she had the option to say, "Oh, I just threw that sign in the window. I need to edit it.
It was from a prior sale for a $2,500 car. This one is a different price."
She didn't. She didn't even ask, "Why do you want to buy it?"
Car offered for sale in comport with her circumstances.
Offer accepted and car bought in comport with the garage owner's.
No gun to anyone's head. No lie or obfuscation involved.
Does having a donor engine make his acceptance of the offer dishonest? I don't think so.
Can an argument for "disingenuousness" be made? Sure but it'd be immaterial.