They wouldn't need to believe he was buying it for just the pudding. They would need to believe that the donated pudding had a certain monetary value, that this guy then donated. And that value would be natural to assess against the cost of acquiring the pudding when the donation was made to someone for the benefit of the food..
That he could extract additional value through the work of removing the bar code does not implicitly reduce the value of the pudding itself.
It is not unusual for it to be possible to extract more value by splitting up a product and selling or taking advantage of parts separately. A lot of businesses are even built on buying lots and separating them to extract value as people are often willing to pay over the odds for a smaller part, or parts of the lots may have been undervalued (as clearly was the case here).
That he could extract additional value through the work of removing the bar code does not implicitly reduce the value of the pudding itself.
It is not unusual for it to be possible to extract more value by splitting up a product and selling or taking advantage of parts separately. A lot of businesses are even built on buying lots and separating them to extract value as people are often willing to pay over the odds for a smaller part, or parts of the lots may have been undervalued (as clearly was the case here).