Obviously, the best way to fix this is to prevent it before it ever occurred. However, it seems like you don't have this option.
Selecting edges and then cloning into them with the "darken" blending mode certainly is feasible for this image because the edge(s) in question are fairly simple. However, if the haloed edges are more intricate (eg, around leaves in a tree), I essentially automate the above process as described below.
First, desaturate the image and run it through the high pass filter in PS. I usually use a radius of a couple of pixels, ie, around the size of the halos.
The high pass filter takes the difference between a given pixel and its neighbors. No difference is displayed as an intensity value of 128. To remove bright halos, we want to select them. In other words, we only want to process edge pixels that are substantially brighter than their neighbors. An easy way to do this is to run the high pass filtered version of the image through a curve which is zero up to an intensity of 128 and then jumps to 50% and rises linearly from there to full brightness.
You next want to use the pixels surrounding each transparent (ie, halo) pixel to fill in for each transparent pixel. There are several ways to do this, but probably the simplest (but not best) way is generate a mask that will select pixels further away from the bright ones, and then apply that mask to a blurred copy of the original.
To generate that mask, simply blur the 1st mask that we just generated. I used an radius of about 5 pixels to do this. Too small, and it won't fill in wide halos, and too large, and it will be picking up color and luminosity info from too far away.
It will look horrible at this stage because we blurred everything, but don't worry, that will go away in a moment.
Next, use the mask we generated first generated to select out only the areas of the above image that were originally bright halos, and place these replacement pixels on their own layer at the top of the layer stack. Set the "blend-if" sliders for this layer to only allow these pixels to be seen where the underlying layer is fairly bright (ie, where there were bright halos).
As the final step, make another copy of the starting image, and place this on a new layer immediately under the layer described in the previous paragraph.
Doing the above steps, the halo generated by your intentional oversharpening is greatly reduced without introducing too many other artifacts, eg, loss of sharpness in the region of the halo. For halos around leaves and other intricate, geometrically complicated shapes, I find this a lot less work than trying to manually remove such halos with the clone or similar tools.
I know this method sounds complicated, but once you have done it a couple of times and get the general idea, it becomes almost second nature and can be done quite quickly. The most important aspect of it is adjusting the various radii. The attached version is just my initial guess for them. If I spent the time tweaking them, the result would be even better.
HTH,
Tom M
PS - I just noticed that when attachments are viewed in-line in these forums, there are considerable JPG and other artifacts, especially around sharp edges (like the mountain in this image). To correctly compare the different tweaked versions, they must be fully enlarged and viewed at 1:1.
