There really is no legitimate excuse. If they are really having problems getting the money to pay you, that is unfortunate for them, because they can no longer afford to have you performing any more work for them. There really is no grey area there.
Eventually, you will learn to let excuses go in one ear and out the other. If you didn't provide the work and still wanted to get paid, they would NEVER even consider writing you a check, no matter what excuse you gave them. Why should you conduct business differently than they would?
If you don't pay your cell phone bill, they stop providing you with the service. It's really simple and really effective.
