I know it's hard to ask for money, and in a perfect world, you shouldn't have to. But I learned from an old boss that's it's okay to ask. He would say something like, "The total you owe me for the purse is x. I know money is tight, but when do you think you can get it to me?" Get a specific date, and then on that date (if you aren't paid sooner), say, "You mentioned you'd have the money for the purse today, and I was wondering if you have it for me."
He used to say, "What did you ever buy that you didn't have to pay for?"
I don't mean to put the blame on you in any way, but letting people get away with this (and it's often unintentional forgetfulness) isn't fair to you because it makes you feel bitter and burned, and you sound like too generous a person to have to carry that burden.
Just my two cents!