Relieiving order has been supplied and chapter stands closed.
Employee has the option of submitting notice of resignation mentioning effective date of resignation or resignation with immediate effect.It is not clear whether you submitted notice or resignation.
After submitting notice employee either continues to work and attend office unless adviced acceptance of resignation or not to attend office or applies for leave.
It appears you have submitted resignation with immediate effect or without mentioning effective date of resignation, and stopped attending office.
You enquired from company after 15 days and you were asked to complete the formality of exit interview and you were issued releiving letter dated 5th of the month, and time of acceptance 24 hours.
You can ask the company to adjust your earned leave and other wages/reminbursements etc to adjust in your F&F payment.
You can take up the matter with your concerned HR employee/Head and explain that on dated ....you called Mr..........from phone number............at his phone number .since you did not receive any confirmation/acceptance of your resignation for 15days ,supplied by email dated...........and you were asked to come to office on dated........and you did so.However the relieving letter is back dated to 5th of the month and resignation is accepted in 24 hours from its date i.e 2nd of the month.
In all probabilities company shall inform that your resignation was being processed and acceptance/releiving order was to be desptached to you and you were called to attend exit interview., and as per appointment letter notice pay ( as mentioned in appontment letter basic or CTC) is to be paid by you.