Hmm I'm still a bit confused. You did the math when there is one simulation of you and found that he expects to make 20 by giving away money and 25 by not.

If there are two simulations doesn't it go the other way? If your strategy is to give away money there are now four indistinguishable siutations. 1/4(£260+£260-£100-£100) = £80

And if you decide not to give money away there are three indistinguishable situations. 1/3(£0+£0+£50) = £16.67