Is my agreement to record 300 laser pulses instead of 267, in order to reduce the cost, considered Haram (forbidden)? And what should I do if it is?

If the girl is authorized by the owner of the place to reduce some of the rent, then there is no blame upon you. However, if she is not authorized, then you must return the difference between the due rent and what you paid, or seek absolution from the owner of the place.