This may be true for some bosses, but it's not a good general rule. You often don't hear about it, getting an outside offer and a raise happens all the time, and within a month it's pretty much forgotten.

If you present it by saying "I really love it here and want to stay, but the compensation being offered here is really good," then your boss should be fine. If your boss can't handle a business negotiation, then you should be looking to get out anyway.

Bottom line: it's a market, and you have to negotiate. Once the papers are signed, things usually go back to normal.