Building a logic to get data from a table

Hello,

I am trying to figure out how I can write a logic for getting the invoice number from DFKKP-XBLNR but the XBLNR is not a key in the table. I try to get the data from the table but it takes a long time to retrieve the data. I found that this might be an option.

ERDK-OPBEL ===> ERDB-OPBEL => ERDB-INVOPEL ===> DFKKOP-OPBEL