How to apply one receipt to the same transaction multi times via webservice soap api?

Summary:

Here's the scenario: one AR transaction (no installment lines) and one AR receipt, transaction is 1000 USD, receipt is 2000USD. The first application, applied 500 usd to the transaction. Few days after, need to apply 500 more to the same transaction.

On UI, users can edit the applied amount, and save the receipt to complete the application. However, we found it not available through receipt application webservice soap api. The api seems cannot support this operating mode. If users want to apply one receipt to the same transaction multiple times using webservice soap, the only way is to firstly unapply the former application via the unapplication webservice soap api, and then apply the receipt to the transaction via the application webservice soap api, the new applied amount is 500+500=1000USD.