You joke but I actually could see Lille open to accepting installments in exchange for a slightly higher fee (other rumors I’ve seen claimed Gabriel’s fee would be closer to 25 million).
Lille are enjoying a good period of success and should have two straight seasons of CL money. According to transfermarkt, they had a net profit of £30 million last year from transfers, and already sold a £45 million player this summer. They don’t need money right away.
Edit: I am wrong about 2 seasons of CL. Forgot ligue 1 only sent 3 teams to the CL, so Lille actually is head to Europa. Nonetheless, the CL money, transfer money, and now Europa money is pretty good for a club of their stature. I doubt they have wage bill commitments that will require immediate cash.