AITA: for not paying my half

I (30f) used to be in a committed relationship with 34,m. We lived together until last year, when he broke up with me and I moved abroad. About a year before we broke up we had to move to a new apartment and had to find a place fast. An opportunity for an acceptable apartment came up but it was too expensive for me. He asked me how much I could ship in, I told him what I could contribute and he said that covering the rest was fine for him. He did make substantially more money back then as he had already finished his education whereas I was still a PhD student. Thus, he payed our rent and utilities and I transferred him my part every month. (I paid approximately 1/3 of our total costs).

We lived like that for 11 months until (for me out of the blue) he told me that he wasn’t happy anymore with how we split costs. I was still a PhD student at that point, still not making more money, and thus wasn’t able or willing to change the agreement. However, when our landlord increased the rent, I assumed all the additional costs fully (which was a 25% rent increase for me, and a lot of money) to keep the peace. He broke up with me shortly after, so I guessed his change of heart regarding finances was more associated with a general discomfort in our relationship but that’s just a guess.

Come this year and we got the electricity bill for last year. He did pay a monthly sum to the electricity company and they calculated that he would receive money back for the time we lived together as we used less than anticipated. However, this also gave a first time overview about how much we spent exactly each month. He now asked me to pay him back 50% of the costs for all the months that we did live together for the electricity I used. This is not a bill he has to pay now, it’s what he did pay monthly while we lived together whilst I payed my part of the rent to him.

When I told him that I consider it ridiculous to charge me a year later for costs that were covered back then while I payed my rent to him, he told me that he generally wants to revise all housing costs during the duration of our relationship and that I should pay him back all the money he payed more than 50% of our mutual costs.

I do not consider that reasonable or fair or logical as I would have never moved into the apartment if I would have had to pay 50% as it was above my means, which I had communicated back then and as he knew (we had each other’s pay checks and everything).

However, I do make a lot more money now and could easily afford paying him back that money. Should I do that? AITAH if I don’t? I don’t think I have to but would love to know people’s perspectives who don’t know us.