Since (presumably) the journey is over, he can insist that I pay my debt in legal tender - but not on the spot: legal tender is for when it reaches court. So, I would tell him that he can take my Scottish money or invoice me.
They're historical accidents tbh.
> Legal tender has a very narrow and technical meaning in the settlement of debts. It means that a debtor cannot successfully be sued for non-payment if he pays into court in legal tender. It does not mean that any ordinary transaction has to take place in legal tender or only within the amount denominated by the legislation.
http://www.royalmint.com/aboutus/policies-and-guidelines/leg...
https://en.wikipedia.org/wiki/Banknotes_of_the_pound_sterlin...
Group A don't know it's legal tender, so they refuse to accept them.
Group B do know it's legal tender, but also know that group A is quite large. Hence some members (group BA) avoid the currency because they know it will make transactions with group A more difficult.
The remaining people, group BB, also know that group BA will try to avoid the currency. Some of those (group BBA) will avoid the currency because it makes transactions with group BA more difficult.
And so on.
I don't think the way Northern Bank notes now say Danske Bank on them instead is going to help me convince shops around here (non-London England) to take them either...
Last time I was in Belfast I got an Isle of Man fiver in my change - I don't fancy my chances of spending it here but I'd never seen one before so I'm just going to keep it for novelty value.