The €100bn bill was one analysts estimate of the gross liabilities and even that analyst states it would down nearer €55-75bn net (
https://fullfact.org/europe/eu-divorce-bill/)
The whole divorce bill thing is a sticky position as there is of course pressure to tell the EU to go and do one. However, if we want to set up new trade agreements, the image of walking away from one we're in at the moment without paying what we agreed in the last budget round doesn't look good.