Hello Sara
FINT does it in a standard way. I think the Original Invoice no. is stored in Invoice Reference field or some other field in the interest line item. Am 100% Sure about that. Can you check the FI doc posted upon FINT
If you still need it, you can use FI substitution to populate in Xblnr. You can also use Bte 1820 or 1120, I believe. If you press F4 in the available events in Fibf- settings- process modules - of a customer, you can easily identify it
Ajay M