Employees have a contractual term that their salary will match whatever inflationary increases are agreed from a a particualr national payscale body. This normally takes effect in April each year. This year the agreement was not agreed by the payscale body until much later than April so it will be backdated in the December payroll. For people that left in say August are we obliged to send them their back pay?
They worked for you during the contracted period so any ratification to that contract still applies.
For people that left in say August are we obliged to send them their back pay?
That depends on the specifics of the contract and local laws.
In every company where I have ever worked (in the US), back pay would not have been required.
If you want to know the correct answer for your case, you must read the contract and potentially talk with a local labor attorney.