Is it possible to overwrite calculated fields?

Hello Ada Napierala, 

A possible way to do this is to migrate the logic that you currently have in the calculated formula into a SetValue Dependency . 
This will allow you to decide your desired rules to set the field read-only/calculated.

For example, you could use a checkbox field to allow the user to enable the contract value to be overridden, something like this: 

<?php

//path custom/Extension/modules/Opportunities/Ext/Dependencies/handle_contract_value.php

$dependencies['Opportunities']['handle_contract_value'] = array(
    'hooks' => array("edit", "view"),
    'trigger' => 'true',
    'triggerFields' => array('override_value_manually_c'),
    'onload' => true,
    'actions' => array(
        array(
            'name' => 'SetValue',
            'params' => array(
                'target' => 'total_contract_value_c',
                'value' => 'ifElse(equal($override_value_manually_c,false),<yourCurrentFormulaHere>,$total_contract_value_c)'
            )
        ),
        array(
            'name' => 'ReadOnly',
            'params' => array(
                'target' => 'total_contract_value_c',
                'value' => 'equal($override_value_manually_c,false)', 
            ),
        )
    )
);

I hope this helps. 

André

Hello Ada Napierala, 

A possible way to do this is to migrate the logic that you currently have in the calculated formula into a SetValue Dependency . 
This will allow you to decide your desired rules to set the field read-only/calculated.

For example, you could use a checkbox field to allow the user to enable the contract value to be overridden, something like this: 

<?php

//path custom/Extension/modules/Opportunities/Ext/Dependencies/handle_contract_value.php

$dependencies['Opportunities']['handle_contract_value'] = array(
    'hooks' => array("edit", "view"),
    'trigger' => 'true',
    'triggerFields' => array('override_value_manually_c'),
    'onload' => true,
    'actions' => array(
        array(
            'name' => 'SetValue',
            'params' => array(
                'target' => 'total_contract_value_c',
                'value' => 'ifElse(equal($override_value_manually_c,false),<yourCurrentFormulaHere>,$total_contract_value_c)'
            )
        ),
        array(
            'name' => 'ReadOnly',
            'params' => array(
                'target' => 'total_contract_value_c',
                'value' => 'equal($override_value_manually_c,false)', 
            ),
        )
    )
);

I hope this helps. 

André