At what amount credit note to be issued

Hii Experts/ others,

I have a client who is in business of selling goods. Now an instance has happened that, he is required to issue credit notes to few vendors who had purchased goods in Nov, Dec & Jan..

He has collected amt incl of GST..

for eg: One vendor purchased goods worth 59000 ( 50k net amt and 9k gst) in Dec 2019, now he is returning goods worth 25k ( net amt ) than for what amt should the credit note be made??? whether of net amt only or amt incl. of tax reversal???

Also want to ask that what is last date by which a cr note can be issued to a customer/ vendor for a particular F.Y.???

Kindly revert...