I think its fine to tell your neighbors how you feel about what happened. State the facts first. Say something like "On Mon you asked if we were getting the aerator and I said yes. On Tue my husband calls to bring it over and you tell him you can't afford to split. So he brings it over, your husband uses it and and to pay his half..." You see where I'm going with this?
After you go through the facts, end w/something like "B/c we are on a tight budget ourselves (no matter if you really are or aren't) I just want to understand what happened so we both don't end up in the same position again for anything else (although I highly doubt you are going to go halfs on anything and hope you don't w/these people again).
That's letting her/them know what they did w/o actually accusing them of anything.
Your husband is another story. My husband knows not to be taken monetarily advantage of but if he does pull something like this I would tell him "if you feel you have x amount of dollars to spend on other people, then i feel like I am allowed to spend the same amount on myself". I understand his charitable nature and since it was already paid for he might have just wanted to get the most use out of it, but in my book, an aerator is hardly an necessity and their lawn is hardly a charitable cause.
Remember, you are losing sleep over this while they are probably sleeping fine. Give yourself one more day to lose sleep over this and then be done with it once you say something. When it is all said and done, they are most likely not going to change but you can at least have learned a lesson and be the wiser person b/c of it.