「なん」is like the 「のです」that you had encountered earlier. It sort of adds a point of emphasis and it is used for explanation. In this case it is indeed だ＋が, so it goes like だんなの土地なんだ が (it is the master’s land but, were the master to permit it, would you be able to… etc).
Nouns and な adjectives take なん or なの before だ or です while い adjectives and verbs use just ん or の.
On a side note, since I’m not completely familiar with the context, だんな can also mean husband, so if the 地主 is actually a female, it would be “It’s the landlord’s husband’s land but…” etc
Hopefully my explanation kind of makes sense!