I’ve seen both, how to unallocate customer receipt and supplier payment questions here in the community, all answers reference cancelling the matching within the mixed payment to create a parked payment and then using the customer/supplier offset proposal to allocate as you wish.
But you can’t cancel the matching if the mixed payment was done in a previous closed period. And you can’t open an old period up as it’s been reported on and audited.
Any ideas please?