I don't know for sure, but I imagine money from N Ireland is legal tender anywhere in the UK, just as Scottish money is. It probably just looks different because it's been issued by a different bank (Bank of Ireland?)

It is Sterling isn't it?

Northern Irish money is fine, as long as it says sterling. You might have trouble in smaller shops but you could always take it to a bank and get it changed

I know from when I worked in retail we couldn't except Irish notes , wether thats changed now I don't know but as White hart said hun the bank should be able to change them for you.

If it is notes then it is legal tender but, as the others have said, convincing a cashier in a shop of that may be another matter. Any bank or post office should sort it out for you though.

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by Sweet-Sange
I know from when I worked in retail we couldn't except Irish notes

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Northern Irish is legal tender but a lot of places may say they don't accept Irish notes to avoid taking Southern Irish notes by mistake (which aren't legal tender. Even in the Republic of Ireland now because it's all euros these days).

I know this issue came up in the pub the other day, one of the staff was asking about Scottish and Irish notes. We accept any but the £1 notes, so you can always come and spend them here if you can't find anywhere else

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by White Hart
(quotes)

Northern Irish is legal tender but a lot of places may say they don't accept Irish notes to avoid taking Southern Irish notes by mistake (which aren't legal tender. Even in the Republic of Ireland now because it's all euros these days).

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Yup I think that was the main idea as to why we didn't except any , although with a lot of shops now excepting euros Im so glad I don't do retail anymore (well unless I'm doing retail therapy of course)

As other people have said take it to a bank and exchange it (and this is probably the best way of checking there's still no difference between the Irish pound and English pound exchange rate)

If is does appear there's a difference then its up to you to decide whether to accept the loss or whether to contact your buyer and insist he sends a cheque (as you specified) whilst you return the money.