How do you link a credit memo to the sale invoice?

[bobpv]

I thought from reading the data definitions it would be in the Link to Another trx field, but does not seem to be there. Can anyone point me in the right direction?
I just want to subtract the credit form the total sale and display that amount in a Crystal Report, should be simple but is giving me a bit of difficulty!

 

[bobpv]

Never mind, discovered that the user had never linked the CM to an invoice! No wonder I could not get a link.