We had developed a custom solution from scratch for our client. Recently the client asked for a quotation to integrate it with a third party software.
Unfortunately, we merely received a hastily guided demo of that software from its vendor, a demo API endpoint, and some paltry 'documentation' containing examples on how to call the API with no explanation of the data returned whatsoever. We've asked for more documentation or at least demo access to that software so we can 'reverse engineer' the API (kind of silly, I suppose) but unfortunately the vendor refuses to give any of that unless our client signs a year long contract with them.
Given the massive lack of visibility on this, I am hesitant to provide any kind of quotation. However, I was pressured into giving one, so I quoted a high price for it to cover the risks involved.
Of course, the client is unhappy with this and claims he can't afford to pay for it. Furthermore, somehow it seems that vendor has told the client that they can do the integration themselves at a much lower price by modifying our software.
While the code we've written now belongs to the client and they can do whatever they want with it, how do I tell them this is a very bad idea and will most likely fail? Also, the client is still paying us for software maintenance, so we would like to keep the code in a good state and of course, we would not want to be blamed (or be fixing) for any issues that might arise from the vendor's changes.