Ex’s lawyer now demands condo sale/buyout per 2015 deal; after years silent, he seeks full value though mortgage remains ignoring fair deductions owed

I am reaching out for legal advice regarding an issue with my ex-husband. We divorced in 2010, and under our divorce agreement, the marital residence (a condo) was to be sold or I would buy out his share once our son turned 18. Per the agreement, I was solely responsible for the mortgage, while any necessary repairs over $500 were to be split equally. In reality, I ended up paying for all maintenance, including approximately $10,000 in major repairs. The outstanding mortgage balance is about $46,000. The condo was recently appraised at $200,000. My ex-husband is now demanding his “full half” of the appraisal value — $100,000 — without accounting for the remaining mortgage balance or the repair costs I covered. His attorney has sent me a letter insisting that I either buy out his share or agree to list the property for sale. I am concerned that he is attempting to claim more than he is entitled to and is disregarding both the mortgage liability and my repair contributions.