I'm not going to get involved in any personal disputes here, but just for reference here are a few of my comments.
Usually, in a business setting payment is due "net 30" or maybe "net 90." Meaning, of course, payment is due within that many days, however it is not uncommon for people to be late on payments. I know some people who have stopped doing business with businesses simply due to consistantly paying late. Also, many businesses will give a discount for payments which come early. For example, if the payment comes within 7 days there may be a 5% discount.
The reason this is done is due to cashflow. As a webmaster you may be familiar with Google paying 30 days after the end of the month's earning. Because of this, you may be able to say Google owes me $5000, but I won't get it for a month, however I need to hire someone to do X job for me. If you wait for the cash to come you may fall behind or lose out on future business.
I don't know what services/products were bought/sold here, however I would recommend that you request payment up front. Some choose half before the job is done, half after. This will help ensure that you only do jobs that you will get paid for besides avoiding conflicts like these.
For me, if the person has a good reputation I may accept full payment once my service is completed. However, if I don't know them I require the payment upfront.
