You need to withdraw your product/service from consideration by this customer. In other words, refuse to sell to them. Come up with any reason. This guy sounds

You need to withdraw your product/service from consideration by this customer. In other words, refuse to sell to them. Come up with any reason. This guy sounds like your typical corrupt politician or organized criminal. They typically do you a "favour" that you didn't even ask for, one that is executed in such a way as to break some rules/laws however petty. Having preemptively performed a favour for you, they have you on their books for when they want to call on you for a return favour. Even though you don't think you owe them one, the shady nature of that first favour is always a dark cloud hanging over your head. They're master manipulators and blackmailers with a balance sheet of favours. It's simply warped. You shouldn't be afraid that he'll sabotage your bid. You should be afraid that he'll give you the bid anyway, then call you up later saying, "look I did this favour for you, now I need you to do X for me." From there it's a slippery slope. The only way I've found to deal with these folks is not to deal with them at all and deny any favours from them. I wouldn't accept a glass of water from them if I were dying of thirst.