Here is a sentence:
Er war groß und bullig und hatte fast keinen Hals, dafür aber einen sehr großen Schnurrbart.
This one is quite confusing to me because I thought that the so called da-adverbs have to be used in cases when the thing they're referring to has already been mentioned. Hence, "dafür" would mean something like "for that". But I do not understand the grammar behind this case in point.
Using dafür to refer to something already mentioned is only one of several uses of this pronomial adverb. Dafür can also mean anstatt dessen/stattdessen, meaning instead/in place (see, e. g., meaning 4. b) at the DWDS).
Er war groß und bullig und hatte fast keinen Hals, dafür aber einen sehr großen Schnurrbart.
(He was big and beefy and had almost no neck, but instead (he had) a very large mustache.)
Especially in everyday language, dafür occurs in this meaning more often than anstatt dessen or stattdessen, in my opinion.
"Dafür" in the usage here and in Björns usage is not a Pronominaladverb, but a Konjunktionaladverb. Although they overlap, you can clearly see the distinction here:
Ich möchte ins Kino, aber habe dafür keine Zeit. (Pro-Adverb)
Ich möchte zwar nicht ins Kino, dafür aber in den Zoo. (Konjunktionaladverb)
You can tell because you have troubles combining a the junction with the Pro-Adverb, which is not at all a problem when you use it as a Konjunktionaladverb, as junction + Konjunktionaladverb is an easy combination to do.
