But from the point of view of the twitter stockholders they get 100% cash from musk for their shares.
It seems he might not have the financing in place so twitter have doubts about the offer. In theory, he can sell his tesla stock and have the money at hand. It's very unlikely for him to do that.
It would probably take him a while to get more than his supposed net-worth. However, getting $10-15B might be < 24 hours.