I wonder if it would be worth having another discussion with the potential adopter, Deb?
Even if this little girl does completely lose her sight, she could still have a very good quality of life as an indoor cat.
Provided basic care was taken, and no changes of furniture etc, she would adapt very well. Also, if the carer was to put, say, £15 per month away, by the end of a year she'd have £150 towards the possible cost of an op, and she may be able to come to an arrangement with her usual vet about instalment payments. An op may not be needed for a couple of years, which would give the adopter an opportunity to put some more cash away.
I realise it may just be a bit too much responsibility for a potential Adopter, and that whoever takes this little beauty needs to be 100% committed, but I'm sure there will be someone out there who would make a great new Cat Slave for her.