REFUNDED ONLY HALF AFTER THEY MESSED UP

I went to purchase 3.6m of cable that i needed for a job and i paid a large deposit. On collection of the cable i see that they had ordered and charged me for 36m of cable(R3810). i was not rude with them and i was understanding of the situation. I then asked if they could take the long cable of 36m and make me two cable of 3.6m and then refund me for the rest. if 36m of cable cost R3810 the logic would be R105.83per meter, therefore 3.6m x 2 = 7.2m therefore = R761.99. but now they charged me R675 per cable i don't see the logic in that, they inflated the price to double so that they did not have to have to pay me back the full amount that i paid to them, i really feel ********** and that they have no principal as a company and no morals if they ripped all there clients of like this, no one will go back to them. i myself am a business owner and i would never do something so under handed like that to my clients, they might as well be thievies