Step 1: Understanding the Concept:
The question describes a procedural defence available to a defendant in a money suit. When the plaintiff sues the defendant for money, and the defendant also has a monetary claim against the plaintiff, the defendant can ask the court to adjust their debt against the plaintiff's claim.
Step 2: Detailed Explanation:
This statutory defence is known as a set-off. It is governed by Order VIII, Rule 6 of the Code of Civil Procedure, 1908. For a set-off to be claimed:
\begin{itemize}
\item The suit must be for the recovery of money.
\item The defendant's claim must be for an ascertained sum of money.
\item The sum must be legally recoverable from the plaintiff.
\item Both parties must fill the same character as they fill in the plaintiff's suit.
\end{itemize}
A set-off has the effect of a cross-suit, and the court can pass a decree in favour of the defendant if their claim exceeds the plaintiff's claim.
\begin{itemize}
\item Cross-claim is a broader term used in some legal systems, but in the context of CPC, set-off is the precise term.
\item Cross-demands is a general description, not the specific legal term.
\item Cross-decrees refers to a situation where two parties hold decrees against each other, which can be adjusted during execution under Order XXI, Rule 18.
\end{itemize}
Step 3: Final Answer:
The statutory defence of settling mutual debts in a single suit is called a set-off.