When I used to flog it I had loads come back due to the customer ordering too much. Not really much you can do once you've started to cut it and tape it to the floor, used to have to tell the customer they can pay to get it re-delivered (and they still can't use it) or they can come and collect it, but it's useless to me if it's been cut I can't re-sell it.
As a result over time we used to try and work out the customers that sounded a little confused and get them to either fax, email or describe over the phone (some funny stories came from this!) their room sizes and we'd work it out, send the gear out, happy days. Still had the odd customer give us wrong sizes, and to be fair had the odd time we gave the wrong combination of kits etc. But we'd swallow the loss on those and usually use the old kits as demo's or even friends / family jobs.
As above though, don't cut the cable, don't hide it and not cover it with SLC. Best thing to do is pull off the last 2m worth of cable and make the loops slightly smaller (not much though!) and then run the left over around the perimeter as said, not crossing wires though.