Other switches don't work with the \b and \f switches (even the \* MERGEFORMAT switch won't work). For what you want, you need an IF test coded along the lines of:
{IF«Cash_Credit»<> 0 "You were selected to receive a cash credit of {MERGEFIELD Cash_Credit \# $,0.00} which will be mailed to the address we have on file."}

Note: The field brace pairs (i.e. '{ }') for the above example are all created in the document itself, via Ctrl-F9 (Cmd-F9 on a Mac); you can't simply type them or copy & paste them from this message. Nor is it practical to add them via any of the standard Word dialogues. Likewise, you can't type or copy & paste the chevrons (i.e. '« »') - they're part of the actual mergefields, which you can insert from the 'Insert Merge Field' dropdown.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]