I have a problem with a customer. The domain tfgld.amnt has a limit of 15 digits but it is not enough for the customer requirements.

This domain could be modified to support 18 digits?

yes , it is possible - but after that you need to change the same in all the report references to the domain .

it has to be done manually and its a real pain.

Note : Try it in test server - before doing in Live.

The maximum significant number of digits for data type double is 15. Is there a special need the customer has to have more than 15 digits before decimal?

Exactly this customer needs more than 15 digits before decimal.

I tried in a test enviroment but I couldn´t write a record with the modified domain.