bigfatbochur

Why when an eid contradicts a baal din does the eid not become a “chaticha deisurah” to the baal din and become pasul legabey the baal din in another case? (if A claims from B that B owes him money through a loan, and C says it never happened, and later C wants to testify to help A against D, then C should be pasul w/ regards to A and A shouldn’t be able to use him)?