Well I'm not sure but I assume unless there's some weird clause in the contract the payment terms remain the same regardless of what we do with him after - so if (as widely reported) we're doing instalments we just keep paying them even if he's no longer our player.
So for talking's sake (i'm just using indicative numbers to show how it could work) if we are paying them 5 instalments of 6m we just keep doing so. If we could reach a deal that gets us, say 30m cash up front with, say 25m in further add-ons and instalments for a total of 55m we get the cash to play with (minus this year's instalment) and the 3 future instalments out should in theory be covered by instalments coming in- we'll owe 18m but have 25m coming in future payments. So in theory that's 24m cash to play with now and future payments covered by future incomings.
There's also the sell-on fee, and no one seems to know if that's on total sale price or total profit. I think the usual is for it to be on profit and putting a sell-on on total sale would be a bit daft, but then it is Everton. If it's profit (as it hopefully should be) we need to give Lille an extra 5m, if it's total sale an extra 11m, leaving us with either 19m or 13m cash to play with.
Assuming it's logically more likely to be 19m, that's not an enormous amount but of course you can spin that 19m into much bigger purchases by agreeing similar instalment transfers- even paying close to half up front that could be 40m, so a 15m midfield replacement and a 25m forward.
All indicative numbers like I say but it shows how Onana could be flipped now to strengthen the squad and it's something we'd have to seriously consider if an offer comes in.