Case 1
Plaintiff Reuvein made a claim against Shimon that Shimon owes $100 to Reuvein through a loan.
Shimon said: "I never borrowed anything from you".
Reuvein brought two witnesses (Pair A) that the loan happened. Shimon brought two witnesses (Pair B) that the loan never happened (the defendant was with them somewhere else at the time the loan was supposed to have been made). The witnesses cancel each other, and Shimon keeps his money.
The Halacha is that all witnesses are kosher because we have no evidence that either pair isn't kosher. However, one witness of pair A cannot testify with one witness of pair B because one of them is definitely lying.
Case 2
Plaintiff Reuven made a claim against Shimon that Shimon owes $100 to Reuven through a loan.
Shimon said: "I never borrowed anything from you".
Reuven brings two witnesses who say they saw the loan. Shimon loses the case and must pay.
Yet, Shimon can use those witnesses of Reuven, even though according to him they are lying.
Question:
Why don't we say, in Case 2, "Meshave Lei Chaticha D'isurah" (One could forbid something to himself)?