I'm not offering you bitcoin, just saying that if I ever happen to sell it to you, I'll be fine with signing a piece of paper saying that I did, because you seem to be attached to invoices and seem to think that if it's signed it becomes "real".

You don't know what bitcoin is? Do you want a link to the white paper? There's s goodf explanation on investopedia, bitcoin.org and other sites. I also recommend Andreas Antonopoulos on youtube.