What would be the most efficient way to gain relationship from a fail result of after selling?

Good question. If your product or service fails to meet the expectations of client, the best way to deal is - To offer them complimentary services. For example, you can offer an additional functionality which is not part of project. However, it is something that will help client to boost up his business. And for this, no extra cost should be charged. You can offer free support for one month or free maintenance or something like that.You see, it is all about compensating things. If one side has gone down, you will have uplift the other side in order to maintain a balance. It is a simple equation. :)