Is it permissible to keep the excess money mistakenly left by a buyer, while being able to return it?

You are not permitted to take or keep the extra money that the buyer mistakenly gives you. You must repent for taking it and return the right to its owners if you know them. If it is impossible to identify them or reach them, then you should give the money as charity on behalf of a Muslim, and spend it on the interests of Muslims on behalf of a non-Muslim. If you later find the owner of the right, he has the choice between approving your disposal of his money or reclaiming his right.