I have a transaction where Manager has allocated a payment to (what seems to be) an incorrect invoice and would like to find out how this can be corrected.
It would be helpful if you could explain more in details or provide some screenshots of how the Manager allocated the payment to the incorrect invoice.
Manager will allocate customer credits, if they exist, to unpaid invoices, oldest first. You might find applicable information in this thread: Automated payments have caused overpayments? - #4 by jax, or in this Guide: Manager Cloud
Generally, to avoid automatic allocations, assign receipt of a payment from customer to Accounts receivable and the specific invoice.