it's not possible. It's a game-flow decision to make you keep buying armor. So instead of renewing you mercs contract from time to time and buying armor only when you upgrade. They've gone for Buying the mercs and renewing the armor very often. (I've seen some comments of having to replace armor twice in a single mission.)

Though having said that... I've been banned from the official forums for saying that I've downloaded a copy illegally. So don't expect much more answers from me... :compcuppa:

You learned your lesson now?

Do not copy or do not tell it