I have created a Rule with the Rules module to validate the form on submission.
I try to compare the values of two fields to ensure one not higher than the other. But I couldn't find the appropriate Rules Actions to stop the form submission and prompted an error message.
Event: After saving new content of type Article
Condition: Data comparison Parameter: Data to compare: [node:field-price-A], Operator: is greater than, Data value: [node:field-price-B]