Fix #10520 - Discount Calculation incorrect when changing Currency

[serhiisamko091184]

Description

If the user changes the currency on a Quote/Invoice, the generated Group Discount is not always correct.
This can be most noted when changing the Discount Amount/Percentage, the Group amount will become wildly incorrect

Motivation and Context

How To Test This

1. Set another currency.
2. Create a record in the Quotes/Invoice module.
3. On Edit View try to change the currency and check the result of the Group Discount field.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Final checklist

• My code follows the code style of this project found here.
• My change requires a change to the documentation.
• I have read the How to Contribute guidelines.