2 way communication between 2 payment APIs.

We have a website and we need to develop a module that will connect with a credit card vendor's API (NOT PayPal). There is already an API working between the two platforms, it should be a relatively straight forward process to build the module - you will need to develop a 2 way communication between both APIs - you get some info from the Credit card vendor and send some info to them using callback procedure (which already exists), so it should allow data transfer between both API's.

Programming language can be determined.
Additional info will be sent to potential candidates.