Dear Jerk,
I'm so glad you were able to finish off the milk I left in the office fridge. I now have a small container of organic raisin bran that was supposed to be mixed with the milk. Would you like that as well? I'll just leave it in the break room. I'm sure you'll go 'shopping' again soon and pick it up.
I'm glad we have the kind of relationship where we can just borrow/take from each other/just me without the formalities of asking. It's nice.
If you'd like I'll gladly give you the antidote you'll need for what I put in the milk just in case something like this were to happen.
Best wishes,
Jessie