VCL not cache but deliver

[dserwin]

Hi all,
I have question related to Varnish VCL. I want to write vcl which under 
some conditions will not be caching object downloaded from backend but 
only deliver it to the client. I was trying to use vcl_backend_response 
subroutine, but returning (abandon) results in returning 503 Service 
Unavailable to the client. I'm using varnish 4.1.2.

If sth is'nt clear enough please ask questions.

Thanks for help.

  --
Damian