If you built an interface to a commercial system - the interface code should be considered a marketable commodity and can be shared.

First - be sure you own it. If you built it under contract for a client, the agreement with the client should state who owns the code.

Next - the code should have been written such that the interface can be separated from the remaining application logic and the design. You may only want to share some of the code.

Be sure to state, up front, that the code is ‘AS-IS’ - meaning you will not maintain it, and you are not responsible for errors.

Finally, offer it to the interface owners. They may share it with clients. Be sure to get your name associated with the work, so that you get credit.

The benefits are that you’re sharing something of value (helping others), demonstrating your technical skill (the code should definitely be high quality), and associating your company name with these actions. The interface providers gain by helping their clients reduce their development costs.