Should retail places or Fast Food places warn people to pay exact amount if they have lack of change

That doesn't work. I might go for it if they had to round the 5.79 to 5.80. This is unconscionable.

Did they issue you an IOU for the missing $4?

sounds like an innocent but careless error on their part, where a one dollar bill was mixed in with the stack of five's...

That couldn't happen over here, we don't have pound notes any more. Our lowest note is £5.

And they're different colours and different sizes to help the blind.

£5 notes are blue £10 notes are brown and £20 notes are purple.

We never use £50 notes, but they're red.

Pound notes used to be green, but we stopped using those over 30 years ago.

Why do you think this was due to lack of change on their part? It involved cash, not coin.

Time to speak up was at the sale. You got a receipt, right? You should not have moved in line after the transaction until this got corrected.

It also helps to say, "Here's a ten", so you both hear the bill number.