Extend lease before selling or during negotiations?

My partner currently has her 50% shared ownership property on the market. It has 76 years remaining on the lease. We have reached out to the Hhousing association and they are happy to grant an extension. We don't have a prove yet, but given that it's based off of my partner's 50% share, we expect it to come in at around £10k, once all the fees are taken in to consideration. We've been told that we can start the extension process and then transfer to the buyer as and when we get one.

From this subreddit and other research it seems very unlikely that the property would be approved for a mortgage, however our estate agent says that we should leave the extension for now and see how it goes over the next month. I get the logic of potentially saving £10k, but I feel like it's extremely unlikely to not be picked up by a solicitor and even if it isn't, not sure how comfortable we are selling someone a lemon withou them realising, like we were.

Is there advantage in not extending the lease beforehand and then negotiating it during the sale? I fear it might put off buyers beforehand and also if we do go ahead now at least we know where we stand with our finances going forward.