Sell it to a country that has the technology level needed to copy it. Russia and China likely would be interested; they might not want to do business with the Taliban directly, but might be willing to buy them through a middle man, say Pakistan.
A lot of the ability of the Boston Robotics robots is in their software thought which is much easier to lockdown via encryption. The rest is the hydraulic mechanism which is not easy but is the best known part of what the BR robots do.
Concealing algorithms and executable code is actually very difficult, consider that to run, a program must first be decrypted. Also consider the decades long war between crackers and proprietary software creators. Obfuscating algorithms has been the subject of quite a lot of academic research.
Store the decryption key in a removable module or the wireless controller? Then anyone who captures one is stuck with a ball of encrypted code that can't execute. Dealing with securing software and keys against capture is not new for the military.
Also destroying the hdd/ssd is easier than destroying the whole machine. You could even have a secure wipe option if the machine is still able to run it's computer but is disabled mechanically.
Yup, I'm sure the countermeasures you can think of in the time required to write an HN post will be secure against technically-advanced nations' militaries.
Sure and it's a problem the military has been dealing with for decades. I'm going to assume they understand the calculus and some effective countermeasures.
It is fairly common for countries to let other powers take a look at technology that lands in their hands (sometimes before returning it to them). See for example https://en.m.wikipedia.org/wiki/Viktor_Belenko#Aftermath