If it really was 75M in GBP that's around 88.5M EUR... if you assume all bonuses up to 24M have already been paid and the sell-on drops to 10% then that would be a 64.5M profit for Sporting.
So 10% of that is 6.45M EUR and that is just shy of
5.5M GBP for Cov. Would be a very nice wedge indeed... but again... no one credible is actually talking about this Arsenal deal at all in the last month or so and Fabrizio Romano who seems to be a puppet or friend of the agent involved has said nothing since April.
For some context around Sporting's predicament, they just sold their #2 striker Paulinho so Gyokeres is literally all they have left:
Saída de Paulinho para o México e desistência na contratação de Ioannidis deixam ataque leonino manco; Rúben Amorim analisa nomes do plano B
abola.pt
In addition to this, a deal for Ioannidis from Panathinaikos seems to have fallen through, with Ipswich also in the mix there. It's likely Sporting will want to sign a striker immediately if Gyokeres is really going, but they also need at least 2 in anyway or they will be severely limited in attack.