Why do returns take so long?

Why is it they charge me immediately for things but when they screw up and have to credit back my account or I return something it can take up to 3-5 days to show.  Even if I have them credit back my credit card immediately for something they rang up wrong it still takes days to show the return.

There have been times that I have had literally the amount in my account I need to spend and they screw up and then I get overdrawn because I have to wait for the return.

With technology being what it is and the fact that they can take my money immediately, there is not good reason why they cannot credit me back immediately.

This is my daily vent!