Better than you think!
A few 'lay of the land' thoughts:
1. Look, we are going to be squeezed very hard by FFP the coming seasons if we get owners that let ETH spend. This is basically where we are right now without any player sales and if we finish Top 4:
I.e. we could only increase our total costs with about 3k. And that is towards a ceiling that you want some marginals towards.
2. But we will of course (try) to sell players. Historically, we have been worthless at that, but who knows. Maybe that is changing. But lets say that we manage to sell Henderson, Maguire and McTominay for a total of £60m, that has the following impact:
That would result in that we could increase our costs with an additional £63m, or £66m in total. How much much is that? Well if we pay £80m on Harry Kane, give him a 4+1 contract and pay him £350k a week in salaries the yearly cost of that is app. £38.2m, and I would imagine that we would want at least a £10m margin against the FFP rules, meaning that we only could afford 'a Malacia' next to Kane if he get that much as well as a new contract for Rashford.
3. But what happens if we make the transfer you suggested? In so called player exchange transactions, we would be deemed to pay the remaining book value of the player we are trading (51m for Sancho) plus the cash sum (50m) = 101m for Kane. Kane and Sancho's salaries would be a wash, and we would replace Sancho's yearly amortization of 17m with 25m for Kane -- so this swap would only increase our yearly cost with 8m. But Sancho is on extremely high wages (he would make 75% more than Tottenham's second highest paid player).
4. Just my opinion, but I would definitely stay away from these type of transfers. And the argument 'that we did it with Casemiro' isn't convincing. We should be in a great spot moving forward -- why risk it with paying insane money to a rival for a player on the decline (I mean he is, his pressing isn't at all what it used to be) that at best got a few more years left in him? Like how easy will it be to convince our existing players to stay within a reasonable salary limit if we make signings like this?