Invoices that have been allocated on a saved payment, but are then allocated and paid on a subsequent posted payment become marked as stale allocations. Payments with stale allocations cannot be posted. The post option on the transaction is disabled and a message letting you know that there are stale allocations displays. 

Correcting a Stale Allocation Payment

Infusion will identify which payments on the transaction are stale allocations, and give the option to remove them 

Selecting Remove Now removes the stale allocation from the paid invoice and allows those funds to be applied to another invoice or remain unallocated against the customer.

Once all stale allocations have been removed the payment can be saved if needed and posted.