In my experience it is far cheaper to replace nylon web slings than to have them re-certified - effectively they are consumerables.
This conversation came up often when I worked for a company that tested and certified lifting equipment, how many of these slings would pass re-certification as you are still paying for the test regime?

The cost of testing small slings is greater than purchasing new certified slings, also in a previous company that I worked for we were getting these slings with every shipment, we did try the certification track but found that 90% would fail so discontinued the process as you pay for the testing each time.

You can try see if Retex are able to recycle them. https://www.retex.co.nz/products/

In my experience it is far cheaper to replace nylon web slings than to have them re-certified — Stephen Small

The cost of testing small slings is greater than purchasing new certified slings — Don Ramsay

I think the main point of the post has been missed - as I understood it wasn't concerned for the cost, rather they were concern for the environmental impacts of using these web slings in a non-sustainable way (i.e. disposal of the slings after only one use when they are possibly still "good")

but the manufacture and disposal has an impact on the environment and sustainability. — Lee C

I have seen some companies (well not so much as a company policy but some workers) keep these slings and use them for non-load bearing uses - e.g. securing materials together. However the main issue is them getting accidentally mixed in with the lifting gear.

Sustainability wise - I see this more as a supply chain issue rather than a "re-use" issue, as there will likely be a finite limit to how many slings you can re-use before you just end up collecting a large pile of slings that don't get used but still need to be "maintained". Even those workers that I knew that would keep some slings only had a handful at a time and would still bin those frequently to be replaced when the next load of materials was delivered (so not really solving the actual sustainability issue).
From the suppliers' position, as these slings are normally left on site with the load until it is unpacked/split, accepting returned slings they would take on a fair bit of risk as they have no control of how the slings have been stored (or used) when the slings are "out-of-their-hands", which they would then be using to secure more materials during transport. Maybe your suppliers would be on-board with reducing this waste and the associated environmental impact - but I would expect them to not be keen as safety will likely trump environment in this case.

That really leaves you with the "re-use for other purposes" like 's suggestion, or if someone is making up-cycled kids swing sets.

Not sure if this will work for your company but we sent a few personnel for Heights equipment inspection course NZQA19359. They were taught on how to visually inspect the slings, on top of other WAH gear like harness etc. We use custom stickers printed by https://www.nztags.co.nz/pages/custom-height-safety-tags after inspection to certify the sling safe for use every quarterly.

Save money in the long run for not needing to send to an external tester.

Thanks for all the responses and the insights.
The key point is the sustainability and the environmental impact.
I'm going to do some further work looking for a solution and will post how we go.
cheers,