Validation failed for the field - Contact

All We have a field on the Sales order 'Contact'  (SOLD_TO_CONTACT) for which we used a custom API in the defaulting rules to fetch the contact either from Bill To or Ship To sites based on some of the requirements We face an issue when the Ship to and Bill To are from different customers, we are able to fetch the Contact we need to and populate in this field but we are unable to save the order as the Contact is not available in the LOV Any suggestions on how we can save the record in this scenario Thanks