He can't take £2 without asking.
But ...
By handing him the £1.50 item, you are offering to buy it for £1.50
He can ring £2 through the till ... he is making a "counter offer"
If you hand over money which includes payment for that item at £2, you are accepting his counter offer. Deal done.
Well done, Jayne. One house point.
You would have got extra marks if you had mentioned:
(1) the difference between sale of goods law and store customer policy
(2) the possible elements of mistake/misrepresentation.