Do they know you are selling the house? If not, tell them and tell them you don't want to spend money on new Colorbond fences as you will never recoup the money in that area.

Tell them you don't mind spending on a basic fence but also suggest that they wait for the new owner who may want Colorbond.

If nothing else you should be able to string them out until the property is sold.

Noting that the neighbour has already commenced on the new fence without agreement, you aren't liable for the cost anyway - so that's worth point out as well.

Ron