Regaining possession to sell a property - difficult tenant

Possession - Section 8 and Section 21

If a section 21 was issued before June 1st then another issued after June 1st 2021, does that mean that the new Section 21 issued supersedes the original one? The tenant is refusing to accept he needs to leave in 4 months time not 6 months. The house has been sold and he wanted to buy it but it was offered to a cash buyer so now he is trying to sabotage the sale. Any advice? The cash buyer would potentially take the tenant on as his AST expires on 31/07 - or would he be able to remain in the property until 01/10? ...any help would be appreciated. The tenant has agreed to move out on 31/07 but he wants £5000 to move!!