3 people put £10 each in a pot to buy a toaster, they give the salesperson £30.00 and take the toaster , the manager says to the salesman there was a £5 discount for cash and takes 5 pound coins from the till and tells the salesman to give it to them, he follows them out of the store & realising he has no small change he gives each person £1 back and pockets the other £2...so the three people now have only paid £9 each into the pot and 3X9 = £27.00 and the £2 in the salespersons pocket - +£ 2 .00 totalling £29.00 where is the missing pound ?

So the three have paid 9 pounds each and the slaesman has pocketed 2 pounds. The proprietor has gotten 25 pounds. We do have 9+9+9-2, (not +2) which is the amount of money in the till.
Or 10+10+10-5 (in the till). Or 8 2/3+8 2/3 + 8 2/3 in the till + 1+1+1 (back payment to the customers) + 2 for the salesperson.