Had a company out to quote for some work on my roof, and they offered me a £70 saving off a ~£400 job if I paid cash. I tentatively said okay, even though I thought that was a bit odd (I might be a tax man – who just leads with that?).
A little while later I figured I'd probably want at least a paper receipt in case they turn around and say 'you didn't give us enough money', and probably an appointment email/text etc. in case there's any damage I can at least point at them being booked in.
I called them and asked if this was possible and they basically said 'no, the whole idea is there to be no record'. I had a weird feeling and cancelled it there and then.
This might seem like '100% of course you made the right call' territory, but the company have solid reviews on Google, Checkatrade etc. had the branded van when they came round to quote, uniforms, brochures and so forth – it wasn't just some random in a white van.
edit: to be clear: yes, they said the discount was cash to avoid the VAT.