The green pieces of paper folded up in your pocket, the balance in your checking account, your mortgage, and that time you paid for dinner with your friend and she said that the next time is on her. These are all forms of money.
That feels awkward, and Microsoft Word isn't too happy with it either. It's some fragments strewn together - but seems out of place. What's the proper way (or a better way) to handle this for a work of non-fiction?
Make them a list of sentence fragments. Poetic license. They sound better, and it helps each thought stand distinctly.The run-on is a little exhausting to the ear.
The green pieces of paper folded up in your pocket. The balance in your checking account. Your mortgage. That time you paid for dinner with your friend and she said that the next time is on her. These are all forms of money.
You might consider creating more topography in this seemingly parallel list. The green pieces of paper are of course seen as money. They are the money. Calling attention to the obvious will help you make your point when you can connect the rest with a conjunction like but or however. This will improve the contrast of the clear example to the other items.
For me, your sentence it is perfectly understandable. I don't feel it's weird when I read it.
