Payroll solution for a group of employees whose payroll ratio is different from another group

Currently, Client FTE employees payroll ratio is 60:40 breakup based on Guaranteed salary - 60% Basic and 40% Allowances. There is an exception case employee whose salary break up needs to be adjusted with a deviation to the above break up rule say 80% Basic and 20% Allowances.

1. All FTE employees are tagged into same Payroll Relationship group (FTE salary).
2. All FTEs will be tagged with Guaranteed Salary salary basis
3. Guaranteed salary break up for salary component (Basic) has impact on the End of service calculation and Pension calculation

Client does not agree to create a separate payroll relationship/salary basis for this group of employees. Looking for any available solution to handle this case