Long story short:
Persons X and Y agree to go on holiday. They find a deal they like, at a price they agree to per person. Person X offers to book it, and person Y pays them back their half.
A few months later it turns out person X had 15% discount code on the total booking. Fair enough, the price person Y paid them
was the going (and pre-agreed) rate for the holiday... but AIBU in thinking that was some grade-A CF-ery by X basically claiming ~£150 cash back from Y?