Is it permissible to donate a debt amount that has been written off by the company? And is it valid to donate an approximate value for a debt whose true amount has not been ascertained?

The fundamental principle is to give the right to its owner whenever possible. If it's impossible to deliver it to them and they haven't relinquished their right, then it can be given as charity on their behalf to the poor and needy. This is the view held by Ibn Taymiyyah and others. This applies to money whose owner is unknown or whose amount is unknown; it should be given as charity or spent on the welfare of Muslims, and one should disburse an amount that is most likely to clear one's liability. It is obligatory to search for the authorized person in the company before resorting to giving charity.