What is the ruling of Islamic law regarding withdrawal from a partnership for building an apartment block, after agreeing on a profit-sharing percentage, followed by the demolition of a floor from the building and the halting of construction after the withdrawal?

If you ended the partnership before the additional floor was demolished and construction ceased, then you are not liable for anything. The entire responsibility falls solely on the owner, because losses are borne by partners in proportion to their capital, and you are no longer a partner. Just as the profit would have been his alone, the loss does not affect one who exited the partnership before its occurrence. The Messenger of Allah, peace and blessings be upon him, said: "Al-kharāj bi al-ḍamān (profit is due to liability)," meaning that the returns and growth belong to whoever bears the consequence of damage and destruction. From this, jurists derived the principle: "Al-ghunm bi al-ghurm (gain is due to liability)." The partnership is terminated by evaluating the land and the building at the time of withdrawal, and giving the exiting partner his share. It is permissible to agree on something else.