Sadly you were technically in the wrong, you gained from the transaction. However, if your line manager knew about this and encouraged it and also didnt explain the company policy to you. Then you might have recourse for him to be hauled in the office. You sound a little young. I would spend a little time thinking about what was said to you about this proceedure and write it down, dates, times if you can remember. I take it your co-worker has also had the same problem. Sadly they might want to make an example of you, but being honest and owning up to you and stating how sorry you are. The good thing is they havent sacked you yet! which means they are lookign into your case. I'd get some legal advice from acas, which is a union.