My (five year old) son gets £1 a week pocket money, and usually saves it up. This week we were out shopping so he was allowed to spend it. He bought himself some sweet popcorn, but didn't immediately open it. He went on to be a bit naughty, so DH decided to confiscate the popcorn.
DS went on to be more whiney and naughty later (unrelated to popcorn confiscation) and DH told him the popcorn would be thrown out if he does not behave. I don't think this is fair - DS bought it with his own money. Doesn't taking it away teach him confusing things about personal property?
I think its different if he has paid for it as he feels it belongs to him properly. I prefer to withhold things like TV, or treats bought for the family. Tbh the stuff he 'owns' is mostly clothes, teddies, puzzles, books - not really confiscation material.
I wouldn't confiscate stuff bought with pocket money, perhaps just say if u don't behave no pocket money next week? I presume picket money is dependant in good behaviour so this week's popcorn is a reward for last week's good behaviour so you can't take it away, the same way you don't take stickers off reward charts....