Why would my ex do this?

My ex ended our relationship a couple of weeks ago. We've only kept in touch on the basis that I owe him some money on a trip we had planned before we broke up (the trip was in my name so I handled all the cash).
I only received the refund on the trip today. I messaged him and asked for his bank details to send the money back. We lost the non-refundable deposit which was £65 each. I said I would transfer him back the money he sent for his half of the main payment (£200). He then told me to deduct his part of the deposit from the £200, making it £135 to send back to him. I asked him why, and he said it 'just seemed fair'. I was a bit affronted by this seeming like 'pity money'. I don't know. He was also pretty cold in the conversation, but so was I then again. We're both being quite stand-offish emotionally and not talking about that.
Why is he being cold and offering his 'pity money'?
Why would my ex do this?
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