Volca Sample button not working? Warranty?

[klendroid]

Hi there! I had a Volca Sample for about a year, very rarely used it, brought it on vacation this summer, played with it a bit, but decided I could sell it, as it just did fit my style of playing.

Anyways - I sell it to a guy, who has now had it for almost 3 weeks, and yesterday he sent me a message to tell me that "button 4 does not work" - only to tell me an hour later that it did work afterall, but today I got a new message saying it doesn't work. Which I certainly never experienced, and I've even done searches with Google to see if anyone else has experienced this, but I can't find a single other instance of a button on a Volca not working.

He's asking me about the warranty of the machine. I tried looking in the manual, but haven't found any info on this. I bought it in July 2015 - if it turns out that one of the buttons of it doesn't work, will it still be under warranty if I can prove that I bought it new in July 2015?

Any help on this would me much appreciated.