Hi,
Where do you maintain the fund, in asset's master data? In this case, you would need to define a derivation rule, which would extract this information: it won't flow automatically just from the fact of posting on this asset. You can use rule of type 'function module' and use pre-defined module FMDT_READ_MD_ASSET, filling GEBER (fund) from the source field.
Regards,
Eli