My understanding is Villa have an option rather than an obligation to buy him back if (when) we go down, same with the other lad they sold to Burnley. They may well exercise that based on his potential, but I can’t imagine they’d purposely tip themselves over FFP to do so.
If they opt not to, I’d expect other clubs to come in. Villa can’t stop us selling to another club or stop Archer choosing to go elsewhere, there is just an arrangement that if we go down we are forced to accept a bid from Villa of whatever the pre-agreed price was. If they don’t want him we can ask other clubs for more (not saying they would pay it!!)