I making online payment service.
I allow users to pay me (merchant) using their credit card. I successfully integrated and tested this feature with the AIM method.
Now, I want to be able to pay users (send money to their credit card) using my bank account (merchant account). What method and APIs should I use ?
You can void transactions or issue refunds or even unlinked credit, but that should all be in the context of previous orders. Sounds like what you're looking for is money transfers, which is not something Authorize.net - or any merchant system, for that matter - is really designed to do. You need to talk to your bank about ACH (Automated Clearing House) money transfers, one of their bigger branches may be able to set you up.