That is called Reimbursement case. Your workshop will issue receipt of cash receiving and write "Party payment" over receipt so that reimbursement amount cheque will come on your name.
You have to submit that invoice to your iñsurance company via surveyor as well as direct submit copy of it to your branch too for fast processing.
I am personally doing it for my cars since last 25 years 
Never forget to keep multiple copies of it.
You have taken your car to the workshop which does not deal with your insurance company directly and thus they are telling you to pay cash and then you will apply for reimbursement.
Usually if documents are complete, then it takes 15 days to 60 days for your cheque of reimbursement amount depending upon your insurance company as well as regular follow-up with your branch.
Stand on their heads so that they cannot delay it.
Finally after having combo of plus minus means lots of positive and negative experiences, i will always suggest new users/ non experienced owners to deal with those workshops only which deals with your insurance company otherwise just lack of 1 document like photocopy of your driving license or any other document or non submission of salvaged/damaged parts will delay your payment for long.
Anyways i am not insurance guy myself but user/client since long era so can guide you from client/user perspective.
You can inbox me anytime for any query.
Secondly, we have insurance Guru Abid bhai....@abid110 here on pw, he can also add his input.
