A person ate meat and milk of fowl (which is Rabbinic). Is he disqualified as a witness?
Eating fowl and milk is a Rabbinic prohibition, not a Torah-level one punishable by lashes. Therefore, the disqualification is Rabbinic in degree, not Torah-level.
Question 2
A person lent money at fixed interest (ribbit ketzutzah). Is he disqualified?
Fixed interest (ribbit ketzutzah) is a Torah-level prohibition. Both the lender and the borrower are disqualified from testimony by Torah law.
Question 3
An acceptable witness knows his co-witness is wicked. He believes his own testimony is true. May he testify?
Exodus 23:1 prohibits joining with a wicked person even in true testimony. By testifying together with the wicked person, the acceptable witness enables the wicked person's unacceptable testimony to be received.
Question 4
A professional gambler (sole occupation, no other trade) testifies in financial court. What is his status?
Gambling as a sole occupation is classified as 'asmachta' — a quasi-robbery since winnings involve no real exchange of value. This is a Rabbinic (not Torah-level) disqualification.
Question 5
A thief steals and then immediately returns the stolen item. From what point was he disqualified as a witness?
Disqualification takes effect from the moment of the transgression, not when consequences are imposed. Returning stolen property does not undo the period of disqualification — it begins when the theft occurred.