If they are trying not to end up in the situation where you go out of business and they are screwed, offer them to escrow the code for a certain amount of money. If you go out of business, they get the code. If you do not, they don't
The $50+bn company might even be the potential buyer, but under code escrow they just get it for free (less whatever was negotiated up front.)
Structure it as 1 year, 2 year, 10 year whatever the hell deal you want. If you are a $50BN company ( you aren't ) you simply fire that customer.
I would think the positive of a large client greatly outweighs the liability of the code escrow.
We pointed to (one of many) conditions that said that escrow would be only released to customers who were in good standing with us -before- the escrow event (and that wasn't transferable, as in they couldn't settle accounts with trustees, acquirers or the like - similar to 'not being able to buy retroactive insurance').