I can't make head or tail of that sentence (poor you having to deal with all these contracts) but 'credit against' means, for example A owes B £100. Unfortunatly B already owes A £200. Therefore A credits what he owes against the amount B owes him, so will now only expect £100 from B.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.