Using PPR Payment approval to approve single/Ad hoc payment
approve single/Ad hoc payment from create a payment or pay in full/ad hoc payment is not supported: https://docs.oracle.com/en/cloud/saas/financials/23d/faipp/how-you-set-up-payment-approval.html#s20057077
1) Create a Pay Group called Single Payment and then Create a Payment Process Request Template for this Group only. Then update the invoice which requires ad hoc payment in the managed installments with the pay group, Then run the PPR which is based on the PPR Template with the Pay group. This can be used in case you need to approve and pay only this invoice. However, make sure that no other invoices fall under the same PPR criteria.