Advice on separation agreement

My (ex)boyfriend and I co-own a apartmenttogether 50/50. We broke up and we are working on the arrengements on separation of the apartment. We were not married and we do not have a samenlevings contract or anything like that.

He was never interested in keeping the apartment, but I do. We reached an arregement on the amount needed to buy him out, which I will be putting together with my savings and a loan of the bank. We put this in a signed (informal) letter, as requested by the bank and notary, since it was needed to draft all the documents. On the meantime, he moved out and I have been paying the mortgage (and all costs) on my own for the last 6-7 months.

However, now he wants to add an extra clause that if I sell the property within the next 2 years, he would still be entitled to 50% of whatever profit comes out of it. He says it is to be sure that I will keep living in the apartment and not sell it for profit. I want to keep living on the apartment and I don't have any intention to sell, but you never know what might happen in the future.

I am asking for advice. Should I just go with it to make the arrengements go quick and smooth? Is there anything that I should be aware of this? I am worried this might bite me in the ass. Thx