Make lookup or rollup field editable based on condition

[Bharathy]

I just created a budget table, and three fields(cost, products, and source) are integrated with the product table  (pulling the cost field from the product table when I choose a particular product from the budget table). if the cost field is empty or 0 in the product table, the integrated cost field in the budget table should be editable. But I have no idea since the cost field type is lookup or rollup. Is that possible?

[TheTimeSavingCo · Accepted solution]

Nope, that's not possible.  You could try:
1. Create a "Number" field in the "Budget" table that you can update the cost for
2. Create a formula field in the "Budget" table that will display the value from the number field if it's filled in, and if not to display the lookup field value