Perhaps you have some scenario you haven't detailed, but this typically isn't a feature you want to allow against posted invoices because of the GL implications.

With miscellaneous cash, you are specifying the offset account that represents the credit portion of the transaction. The debit then will increase your cash account and the credit will be posted to the account you specify. There is no customer involved in this type of transaction so it allows you to recognize cash sales, payments or adjustments recorded in the AR subledger.

The problem is that an invoice is a receivables document that has already been committed in the system against a specific customer and is represented in GL by an increase in your AR assets. When you pay that invoice, you are reducing that customer's AR balance which credits the receivables account and debits the bank account, recognizing the receivable as cash.

If you were to change the account that is credited in the receipts process, it would render your AR subledger out of balance with the AR account(s) in GL. That would force the GL team to create balancing entries to bring the accounts back into alignment. This type of change also breaks some GAAP rules by altering the audit trail and requiring unrelated adjustments to realign the account balances.

Most financial auditors would red flag these types of transactions - unless they have supporting documentation - to ensure they are not being recategorized for nefarious purpose.

Best accounting practice is to post the Customer Payment against AR-Trade to enable you to monitor your Trade-AR.   Miscellaneous Cash Receipt is exempted from this because Miscellaneous may not be coming from regular customer, thus an option to select other GL credit account is open.     

If GL Credit field for Customer Payment will be opened, this may lead into Payments that cannot be applied against Invoice, thus AR-Trade will need a lot of adjustment to reconcile it with the GL. 

This is the reason why the Proforma entry for Customer Payments were fixed into Debit Cash and Credit AR-Trade.

Best accounting practice is to post the Customer Payment against AR-Trade to enable you to monitor your Trade-AR.   Miscellaneous Cash Receipt is exempted from this because Miscellaneous may not be coming from regular customer, thus an option to select other GL credit account is open.     

If GL Credit field for Customer Payment will be opened, this may lead into Payments that cannot be applied against Invoice, thus AR-Trade will need a lot of adjustment to reconcile it with the GL. 

This is the reason why the Proforma entry for Customer Payments were fixed into Debit Cash and Credit AR-Trade.