Write an API to join between a PoS and a payment application (Java/C++/C#)

We write and maintain a PoS software, for processing sales, stock, reporting etc. Different customers require different card payment processors, sometimes for different countries. We are looking for a third party programming resource that we can use for providing the integration software that provides the bridge between our PoS' payment interface and the payment processor. The solutions will all be 'out of scope' ones where the card data is not handled by our software, nor handled by the software that we need written. The card data will be handled by the card processing software, and we will not touch it. Therefore there is no 'PCI' concern. We are already integrated with a few different payment solutions so the PoS already handles well the calls/responses to payment processors. The payment processor providers also have very complete documentation for their own APIs. We are looking for a partner who can take over the burden of these integrations so that our core programming team is free for other projects (like core PoS development). We like long-term relationships. If the initial project is a success then we would return for future integrations as they are needed (as they always are). For the purposes of testing the developer will need to provide access to an ubuntu server so that we can build/update the database for our PoS system. In this way the developer will be able to run and test their own software with our own PoS system. At the end of the project all code will be delivered to us, and stored in our code repository. It will belong to us, although as part of the relationship we would likely return to the same developer for future integrations and any code changes as APIs evolve. The code needs to be written in either Java, C++ or C#, and will be checked for good coding practices internally by us. The final payment will be released once the code is both operational and the code behind it has been approved by our own development team. No GUI is required - this software is effectively translating between the calls/returns of one piece of software and similar calls/returns of another.