Don't be fooled by the negative sign, it doesn't result in a negative number, but rather a very small number. Here's how it works:

-40 decibel milliwatts
= -4 bel milliwatts
= (10 to the power of -4) milliwatts
= .0001 milliwatts
= 100 nanowatts

You can never get decibels to result in a negative number, never ever.

Going the other way, if you wanted to convert the power of the Alfa to dBm, it'd be:

500 milliwatts
= 2.69897 bel milliwatts (I got this figure by taking the Log10 of 500)
= 26.9897 decibel milliwatts
So it's about 27 dBm