When setting up your Zoho Finance integration you map your fields.

If you have custom currency fields such as "Cost Price" setup in CRM these fields will not sync to Zoho books and you'll get errors because the "Field Type" does not match that of Zoho Books. However the Unit Price system field in the CRM will sync fine even through that field is a currency field!!

Workaround: Duplicate your CRM currency fields and re-add them as Decimal field types. Export your data, rename your column to the new currency one and re-import doing "update only". You can then hide or remove your original CRM currency fields. Your records will then sync with Books

I guess the logic behind this is the differing currency types???