Ideally, record the dialogue with another person at natural speed!
If it is not possible, please, record both parts (don't say 'shop assistant' and 'customer'!). If I get several recordings, I will 'reconstruct' the dialogue with Audacity! Thanks for your help!
Shop assistant: Hello, can I help you?
Customer: Yes, have you got this shirt in other colours?
Shop assistant: We’ve got it in black, blue, green, red and yellow. What size would you like?
Customer: Small, please.
Shop assistant: OK, in small we’ve got black, green and red.
Customer: All right, can I try the red shirt on please?
Shop assistant: Yes, of course. The changing rooms are over there.
Shop assistant: Is it OK?
Customer: Actually, the shirt is a little too tight, too small. Can I try size Medium, please?
Shop assistant: Sure. Here is a medium.
Shop assistant: Is it OK now?
Customer: It’s perfect. How much is it?
Shop assistant: It’s £12.99.
Customer: OK, I’ll take it.
Shop assistant: Would you like to pay by credit card or with cash.
Customer: With cash, please. Here’s £20 pounds.
Shop assistant: Thank you, here is your change and your receipt. Have a nice day!
Customer: Thanks, goodbye!
I just listened to that recording by Grooveygreg and it was so amazing I couldn't possibly do another recording.