Hello JP,
material document and Invoice are referenced to PO so there is no direct linkage within both.
But this is the way you can try for and which will be almost accurate.
RBCO- BELNR, BUZEI Pass to RSEG - BELNR, BUZEI and pick up EBELN & EBELP.
Pass EBELN EBELP into EKBE along with VGABE = '1' you will get the mat doc and item.
Regards
Gajesh