Quick tip from Parent Hacks that would work for office personnel too:

Our nanny does a lot of our food shopping for us. It's something for her to do with the baby, and she likes helping out. Usually, I give her a chunk of cash that seems like enough to cover things, and then she gives me the change along with the receipts. Last week it occurred to me that I should just pick up gift cards for her to use! She usually goes to Trader Joe's and Whole Foods. Both of these places sell gift cards (as do most supermarkets). I can put a big lump sum on each card, and then she no longer needs to worry about keeping my cash separate, etc.