The transfer market will never be the same again. PSG inflated it and now no clubs are going to be willing to go back to Neymar prices. Most clubs are still losing money and no one's going to be filling grounds again for the whole of next season so I'm not sure where you think clubs are going to have over £100m to spend next year.
In the transfer market that saw Dembele moving clubs for 140m, Sancho would be easily worth 170. Sancho is better and British, after all. So 120m is not the price in a market flooded with money.
How are clubs going to plug the gap in revenues in that time? Even if there's a vaccine in 5-6 months, they've still lost half a years worth of gate receipts. That's also implying that numbers go back pre-covid highs, which is also debatable. Also, there may not be a vaccine by then! If anything, now is the best time to sell if you want a lot of it up front, I don't see it getting better any time soon. Not to mention Dortmund's model relies on selling and fans in the stadium.
There are a few factors affecting the income right now. First and foremost, empty stadiums. Smaller clubs are affected more seriously by that since incomes through ticketing make up a smaller share of the revenue of the elite. Then there's also the fact that fans right now have less money at there disposal which is obviously affecting merchandising sales. And third, there are less European cup games to be played, leading to a decrease of broadcasting revenues.
I don't think top clubs are really in danger of not being able to pay their employees anymore. Their financial substance isn't shrinking but at the same time the missing incomes through ticketing means that they're also on a tight budget for transfers since they have to cover the expenditures they have. Once that is sorted out they'll have much more leeway. I mean, Barcelona exemplary generated more than 1 billion in revenue recently. It's not like they need to save money throughout a few seasons in order to sign a player worth 100+m. They can stem such a transfer on top of a few smaller ones every window.
So yeah, it's possble that transfer fees remain a little bit lower. But then again, as mentioned above, a player like Sancho would've been closer to 200m a few years ago than to 100m given what was paid for Dembele, Felix, Mbappe and Coutinho.