Opportunity Amount Field in RCE

We do NOT use standard, out-of-the-box opportunity amount field in SFDC and RCE syncs with this standard SFDC "Opportunity" field.  We have a custom field in SFDC that is formula based and populates global currency into USD. I called Marketo Support and they re-mapped the MKTO opportunity amount field to this new custom field.  No luck - I believe MKTO can NOT read opportunity amount field that is formula based. 

I'd appreciate if anyone can share how to set-up a custom opportunity amount field in SFDC that MKTO RCE can read.  Thanks!!