How to make it so that the final price is never in the red, but only 0 in a VM with custom fields?

Question

Joomla

Hello!
I can't find the right php page where the value of custom fields affects the final price. The bottom line is that the product has a price, but in custom fields we can change it as we like, but what to do if the discount is greater than the price, the price goes negative, but this is not possible. It is necessary that the price tag in this case is always equal to zero.
Please help me where to write this condition.