If you get another offer, bring it to your employer and they don't match it or at the very least come up with a counter you both agree to, then you need to quit

If you get another offer, bring it to your employer and they don't match it or at the very least come up with a counter you both agree to, then you need to quit and take the other offer. That's the risk of this move. As you say, you will be viewed as a lame duck at the very least and possibly even get fired. Even if your company agrees to match, they may see you that way and have plans for you to leave as soon as you transfer all your knowledge. This is a power play and it can backfire so it's only advisable to do this if you're ready to accept the other offer and move.