Is it permissible to take a commission on buying and selling currency with an increase in the prevailing exchange rate, without affecting the employer's profit margin?

If your employer permits you to take a share of the profit, then there is no harm in it. Otherwise, it is not permissible for you to take any profit without their permission, even if you sell for more than the price they specified for you. This is because you are their agent, and you are not authorized to act except within the bounds of what they have permitted you. Taking any profit without their permission is considered deception and betrayal. This is supported by the general meaning of Allah's saying: (وَلاَ تَعْتَدُواْ إِنَّ اللّهَ لاَ يُحِبُّ الْمُعْتَدِينَ) "And do not transgress. Indeed, Allah does not like transgressors." And by the hadith of Urwah ibn al-Ja'd al-Bariqi, that the Prophet (peace be upon him) gave him a dinar to buy a sheep for him. So he bought two sheep with it, then sold one of them for a dinar and came back to him with a dinar and a sheep. So he supplicated for him for blessings.