Need to wait for asynchronous response in validateField client-side function

I have a situation where we want to check the validity of a field on a Sales Order record against a field on the Customer record. Both fields hold arrays of internal ids of contact records. So far, so good. If all the contacts selected on the Customer record are present in the field on the Sales Order record after it is edited, then the field is valid. If one or more is missing, then the field is presumptively not valid, but the user needs to have the option to override.

Problem: dialog.create returns a promise object, so starts its own asynchronous thread while the rest of the validateField function carries on to its conclusion. I need the validateField function to wait for the response from the dialog.create promise object before continuing.