The former Portsmouth boss revealed that his only form of monetary compensation is a bonus, which he plans to return if the Blues drop into League One.
"I'm not being paid. It doesn't bother me though," Redknapp told BBC Sport. "I have got a bonus. It's not a massive one, but I said 'if I don't keep you up, I don't want paying'."
Redknapp went on to say that he struck up a similar deal with Derby County when he served as an adviser there last season, promising to help the Rams secure a spot in the playoffs.
"I said to them 'if you don't make the playoffs, I don't want any money'. If we hadn't made the playoffs, I wouldn't have got paid anything," he added.
Birmingham are two points clear of the dropzone with three matches remaining.